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Πατροκλος φιλος ην Αχιλλεως. 16. Kupov, Tov Twv lepowv angle, and therefore a body will travel down it with a greater βασιλεα, επι τη τε αρετη και τη σοφια θαυμαζομεν.

velocity, and it is found that this increase of speed exactly

makes up for the greater distance. EXERCISE 34.- ENGLISH-GREEK.

The other fact is, that if a body has to fall from one point 1. The flocks follow the shepherd. 2. The king has care of to another not in the same vertical line, (for) the citizen. 3. Ear3 are tired by the idle talk of the old as, for instance, from D to E, the line woman. 4. An old woman is talkative. 5. The shepherd leads of quickest descent is not along the the herd of oxen to the city. 6. Oxen aro sacrificed to the gods straight line joining these two points, by (oto with gen.) the priests. 7. O priests, sacrifice an ox

but along some curve, as D F E. The to the gods. 8. Children love their (the) parents. 9. Parents reason of this is, that if the body be aro loved by their children. 10. It is the business of a good moving down the curvo it will at any shepherd to take (have) care of his herds.

moment be at a lower level than it 2

would if falling down the incline DE; KEY TO EXERCISES IN LESSONS IN GREEK.-IX. and since the velocity of a falling body, EXERCISE 25.-GREEK-ENGLISH.

as we have seen, depends upon the ver1. The ravens croak. 2. Avoid flatterers. 3. Keep away from the tical distance passed over, its velocity

Fig. 100. deceiver. 4. Men delight in the harp, in the dance, and in song. 5. is all along greater. The space passed Horses are driven by whips. 6. The harps delight the minds of men. over is, however, greater too; but this is more than compensated 7. A grasshopper is frieudly to a grasshopper, and an ant to an ant. for by the increased velocity. The curve of shortest descent of all 8. The shepherds sing to their pipes. 9. Among the Athenians there is found to be that which has the greatest curvature without were contests between quails and cocks. 10. The shepherds drive the rising as it approaches E. If a pencil be fixed so as to project flocks of goats into the meadows. 11. The life of ants and quails is very laborious. 12. Many have a good countenance, but a bad voice.

horizontally from the rim of a wheel, and made to trace a curve

on paper while the wheel is rolling on, it will be exactly that of EXERCISE 26.- ENGLISH-GREEK.

shortest descent. As we shall see further on, there are other 1. Φευγω κολακα. 2. Κορακες κρωζoυσι. 3. Τερπεσθε φορμιγγι. 4. Ορχηθμοι | remarkable and important properties possessed by this curre, τους ανθρωπους τερπουσι. 5. Ελαυνουσιν ίππους μαστιγγι. 6. Οι θυμοι των

which is called a Cycloid. (See Lessons in Geomotry, XXIII., ανθρωπων ελαυνονται φορμιγγι. 7. Αί συριγγες τερπουσι τους ποιμενας. 8. | page 309.) Αί αιγες εις τον λειμωνα ελαυνονται, 9. Ο ποιμην δει προς την συριγγα. 10.

PROJECTILES. Καλην μεν ωπα εχει η θυγατηρ, κακην δε οπα.

Having thus seen the laws which govern the motion of falling EXERCISE 27.-GREEK-ENGLISH.

bodies, we pass on naturally to notice the movements of pro1. The birds sing. 2. Favour bezots favour, (and) strife (begets) jectilos. Here, of course, as before, the resistance of the air

impedes motion to a greater or less extent. This resistance strife. 3. We count youth happy. 4. Need begots strife. 5. Rich inen often conceal their baseness by (means of) wealth. 6. O fair boy,

increases as the square of the velocity, for if the speed of a love your good brother and your fair sister. 7. Avarice is the mother body be doubled, it not only has to displace twice the bulk of of every kind of baseness. 8. The poor are often happy. 9. Wisdom air, but it must do it with twice the velocity, and for this a fourin the hearts of men stirs up marvellous longings for the beautiful. fold force is needed. As, however, our calculations would be 10. Death sets men free from their cares. 11. Friendship springs much complicated if we took this into consideration, we will up by means of resemblance (in disposition). 12. Wine creates neglect it, but we must remember to make allowance for it in faughter. 13. Deliberation comes to the wise in the night. 14. The

our results. wise punish baseness. 15. Men often delight themselves with light

The path of a projectile is in a curve called a parabola, that (or vain) hopes. EXERCISE 28.-ENGLISH-GREEK.

is, a curve similar to the one which we should obtain if we

were to cut a cone in a direction parallel to one side. (See Los 1. Ορνιθες αδoυσι. 2. Χαρις χαριν τικτει, ερις εριν. 3 Σοφια εγείρεται εν

sons in Geometry, XXI., page 251.) We can, however, trace this τοις των ανθρωπων θυμοις θαυμαστος ερως αγαθων. 4. Τερπομαι ωδη των ορνιθαν. 5. Αί ωδαι των ορνιθων τερπουσι τον ποιμενα. 6. Τερπομεθα ορνισι.

path in a simpler way.

When a body is projected with any velocity, as, for example, 7. Οι ανθρωποι έπονται τους αναξι. 8. Οι ανθρωποι πειθονται το ανακτι.

when a bullet is fired from a gun, it is acted upon by two forces EXERCISE 29.-GREEK-ENGLISH.

-the original velocity with which it was started, which, as we are 1. In difficult matters few companions are faithful. 2. The suppliauts not considering the resistance of the air, we may consider to be touch our knees. 3. Death is a separation of the soul and body. 4. a uniform force; and, secondly, the attraction of the earth, which Wealth furnishes men with various aids. 5. Do not yield to the words is an accelerating force, causing it to fall 16 feet in the first of wicked men. 6. Do not, my son, be a slave to the service of the second, 48 in the next, and so on. Now from a knowledge of body. 7. The Greeks pour cups of milk as libation-offerings to the

these two motions we can easily tell at what point the body will nyaphs. 8. Accustom yourself to, and exercise your body with, toil and sweat. 9. Chatterers vex (or weary) the ear with repetitions (of and by thus finding seve

be at any given moment; the same story). 10. Accustom your soul, my son, to good deeds. 11. Evil stories do not lay hold of our ears. 12. We listen with our ears.

ral different points in its 13. Do not hato a friend for a small fault. 14. My son, taste the milk. course we can trace out 13. The soldiers bear lances.

its path.

Let the bullet be pro-
EXERCISE 30.- ENGLISH-GREEK.

jected from the point a 1. 12 νεανιαι, θιζετε τα σωματα συν πονο και ιδρωτι. 2. Ορεγομεθα των

(Fig. 101), in the direcαγαθων πραγμάτων. 3. Πολλοι τερπονται χρυσω. 4. Εξ αγαθου πραγματος

tion A B, with any given γιγνεται κλεος. 5. Τους καλους μιθους των σοφων θαυμαζομεν. 6. Τα των αγαθων ανθρωπων αγαθα πραγματα θαυμαζεται. 7. Οι στρατιωται μαχονται

velocity, and take ac of λογχαις. 8. Ου διαμειβομαι τον πλούτος της αρετης τοις αναξι.

such a length as to re

9. Mn πιθεσθε τοις λόγοις των φαυλων.

present the space it would
pass over in one second.

Draw A D vertically D
MECHANICS.-XVIII.

downwards to represent

16 feet, and complete the LAWS OF FALLING BODIES-PROJECTILES-COLLISION

Fig. 101. parallelogram

AC and A D represent, then, the two forces acting on the ballet; There aro two remarkable facts that havo been discovered in and, since each produces its full effect, it will at the end of de cornection with the laws of bodies falling down an incline that second have arrived at the point E. Since, however, the force we must just notice here. The first is, that if we take any gravity is not uniform, the line A E, which represents its path, wil number of chords, A E, B E, etc. (Fig. 100), all meeting in E, the not be straight, but curved upwards, for when a half of A e bas lowest point of the circle, and make inclined planes parallel been passed over, gravity will only have caused it to move over a and proportional in length to them, a body will take the same quarter of Ad. If now we draw through E a straight line 3r, time to fall down each of these inclinos. BE, for instance, is parallel to A B and equal to Ac, it will represent the motion or much longer than D Е, yet it is inclined at & much greater the bullet from its original impulse during the next second To

A DEC.

OR IMPACT.

A

B

D

COLLISION OR IMPACT.

reprezent gravity we must take EG, three times the length of AD, Now ruise c to the fourth division, and let it fa!l against D. and thus, by completing the parallelogram, we find that at the No momentum will be destroyed; it will merely be shared end of this second the ballet has arrived at H. In the same way, between the two balls, as much being gained by the one as is lost by making u L equal to five times A D, we find K to be the point by the other; and, since both balls have the same weight, each at which the ballet will have arrived after three seconds, and in will move with half the velocity that c had on striking D. They this way we can map out its wholo path.

will therefore rise to. We see from this the reason why the sights of a rifo are gether to the first divi. arranged as they are. If the bullet travelled in a perfectly sion of the arc D F, for straight line, the soldier would aim directly at the point hoc takes twice as long (E wished to hit; but the force of gravity acts on the bullet, and to fall from 4 as it therefore he has to point the rifle at a point as much above it does from 1, and thoveas the bullet will fall in the time it takes to travel the distance. locit; is proportional If, for instance, it takes two seconds for the ball to reach the to the time, therefore target, he must aim at a point 64 feet above it. To do this it acquires à double would be very inconvenient and uncertain, as he would bo velocity in falling. anable to tell whether the point he was aiming at was directly Now whatever veloover the mark. The sight at the end next the stock is therefore city a body acquires made to adjust to different elevations above the barrel, according in falling from any

Fig. 102. to the distance of the object aimed at; and thus, though the height, it must start rifleman sees the two sights in a straight line with it, the with that velocity to rise to that height. A velocity, then, half barrel is really pointed considerably upwards, as will be evident as great as that acquired by c will raise the two balls to 1. to a bystander.

In the same way, if we make c half as heavy as D, and raising There is one other fact relating to projectiles, which, though it to the 9th division let it fall, the two will, as before, rise to l. it seems strange, is a necessary result of the second law of The mass moved after impaot is threo times that of c, the motion.

velocity will therefore be only one-third as great; they will E a body be projected horizontally, no matter how great its therefore rise the height. We seo thus that when one body velocity be, it will always reach the earth in exactly the samo strikes against another, the momentum will be divided between time as if it fell vertically. The speed in falling is not in any them, and hence the resulting velocity will be as much less than way interfered with by the horizontal motion.

that of the moving body as the mass of the two is greater than its mass.

For example, suppose a ball weighing 1 lb. and moving with a We said that any force is measured by the velocity generated velocity of 60, to impinge against a larger ball weighing 14 lbs. in a second. There is one class of forces, however, which cannot The mass after impact will bo 15 lbs., or fifteen times that of the be so measured, because they do not act for any appreciablo ball; the velocity will therefore be is, or 4 feet per second. No length of time. These we call impulses or impulsive forces ; momentum is lost. The original momentum was 1 x 60; after any force which is of the nature of a blow is placed in this impact it is 15 x 4, which is also equal to 60. class.

This principle supplies us with a means of measuring very When one body strikes against another different results will great velocities, as that of a cannon-ball or other missile. ensue, according to the nature of the bodies. If an ivory ball A large block of wood or metal is suspended by a rod so as to be allowed to fall on a stone slab, it rebounds or rises from its swing to and fro with as little friction as possible. This is aurface, but the height to which it rises is less than that from called a ballistic penduluin. Against this the ball is caused to which it fell. Were the ball perfectly elastic, it would rise to strike, and by its impact it sets it in motion. A graduated arc the same height. This, however, is not all that has occurred, is fixed under the block on which the distance to which it the changes have been more complicated. On striking the slab, swings can be noted, and from this wo can calculate the velocity the ball is first flattened in a slight degree. In proof of this we it had immediately after the ball struck it. We havo only to may smear tho slab with oil, and we shall find the ball marked, measure the vertical height to which it rose, and ascertain the not in a minute point as it would be if merely laid on it, but velocity it would attain in falling from that height, and thus per a space increasing in size with the violence of the blow. we have the velocity with which it started. The particles are thus compressed, but their own elasticity The weight of the bullet and the pendulum being also known, causes them at once to recover their original position, and in we can at once determine t':c proportion they bear to each other, so doing the ball flies up from the slab.

and thus we can ascertain the velocity of the ball from that of The effect, then, varies with the degree of elasticity of the tho pendulum. body. We can, however, only consider the cases of clastic and Suppose, for example, that tho pendulum weighs half a ton, inelastic bodies, not that any substances are perfectly so, but and being struck by a ball weighing 24 lbs. is raised to a height by examining these we shall get at general principles, which can of 16 feet. In falling from this height it would wcquire a then be applied or modified as may be required.

velocity of 32; this, therefore, is that which it had immediately We will first consider the case of inelastic bodies, and well- after the ball struck it. But the mass of the ball is to that of kneaded clay or putty may be chosen as suitable substancos to the two together as 24 to 1144, or 1 to 48 nearly. The velocity experiment with. Wax, softened with oil, will also answer well. of the ball was therefore 48 x 32, or 1536 feet per second.

In making experiments on impact, the best plan is to procure Hence wo see why, if one body strikes against another, the balls of the substances chosen, and, having fastened them to heavier it is as compared with that against which it strikes, the strings, suspend them in such a way that they may just touch greater the effect produced. If we want to drive a large nail or one another.

to strike a violent blow, we use a heavy hammer, for by it we Let us take two such balls, c and d (Fig. 102), of equal obtain a much greater momentum, and thus accomplish the weight, and having raised them to the same height, in opposite work with greater ease. So, too, when we are driving a rail directions, leave them free to fall together and strike each other. into a plank, we place a support behind or hold a heavy hammer Since both fall from the same height, their velocities are equal, against it. Unless we do this the momentum is shared by the and they each have the same mass; their momenta are there board, which yields to the blow, and thus destroys much of the foro equal, and being in opposite directions neutralise cach other. effect. But when a heavy inelastic body is held behind, this, Both balls will therefore, after impact, remain at rest.

too, has to share the momentum, and thus the plank yields In order to mcasure thọ distance through which the balls fall, much less, and the nail is driven more easily. we must draw the arcs which they describe and divide them. In the same way some of the feats of strength somotimes We do not, however, make the divisions equal, but draw a series exhibited may be explaised. A man will lie with his shoulders of parallel lines at the same distance apart, the lowest being supported on one chair and his feet on a second. A heavy anvil even with the top of the balls, and make our divisions at the is then placed on his body, and on this ho allows stones to be points where these cut the arcs. The reason of this is, that the broken or blows to be struck, which, but for the anvil, must Felocity is proportional, not to the length of arc, but to the certainly kill him. The reason is, that the momentum of the vertical height, and thus these divisions indicate the velocity. hammer imparts but a very slight velocity to the anvil, on

£95

40 acres

acres, acres.

account of the greatly superior weight of the latter. This or, what is the same thing, so that the four quantities shall be small velocity is easily overcome by the muscles, which being proportionals. stretched, act, to a certain extent, liko & spring, and thus the It is evident, since a concrete quantity can only be compared blow is scarcely felt.

with another of the same kind (Obs. 11, Lesson XXVII., Vol. II., We must now pass on to consider the impact of elastic page 102), that the fourth quantity determined must be of the bodies, and for this we may take balls of ivory suspended in the same kind as the third quantity. In order that the ratios of same way as those of clay were. We shall find that, though the the two pairs of quantities may be equal, either two must be effects produced by these are different, the same general laws of one kind and two of another, or all four must be of the same apply. The bodios, however, instead of moving on together, kind will, after impact, rebound and fly apart.

2. Suppose we have the following question proposed :Let us raise one of the balls c (Fig. 102) and allow it to fall EXAMPLE. If the rent of 40 acres of land be £95, what will against the other. The first effect will be that the momentum be the rent of 37 acres ? will be shared between the two, but, being elastic, they will be It is evident that the sum required must bear the same ratio compressed, and the reaction in regaining their shape, being to £95 that 37 acres do to 40 acres. equal and opposite to the action, will destroy the motion of c and Hence we have, writing the ratios in the form of fractions, doublo that of D. The former will therefore remain at rest, and Sum required. 37 acres

37

= the abstract nuinber D will move on with a velocity equal to that which chad. If a

40. series of several balls be thus suspended so as just to touch ono

Therefore the sum required 1! * £95, which can be reduced another, and the end one raised and allowed to fall against tho

to pounds, shillings, and pence. others, the motion of the first will be imparted to the second, 3. The last question might also have been solved thus and by that to the third, and so on throughout the entire series,

Since 40 acres cost £95, the motion of each being destroyed by the reaction of the next.

1 acre costs £!; The result will thus be that the end ball only will rise, all the

37 x 95 And therefore 37 acres cost

pounds. others remaining at rest. So, if two balls bo allowed to fall, two will be raised at the other end. We sce, then, that no 4. In solving such a question by the Rule of Three, the statemomentum is lost here, any more than it was in the case of in- ment of the proportion is generally written thus :clastic bodies; but it is not shared between all the balls, as it was in the other case. These experiments can, of course, be varied

40 : 37 : ; £95 : sum required. to almost any extent, and you are recommended to try them for Then, by equating the product of the extremes and means, we yourselves, for more is always learnt by seeing or trying a few exo get the result. We have put the first example, however, in periments than by reading about many. As, however, there is the fractional form, in order to indicate clearly the fact that difficulty in procuring and suspending ivory balls, the experiments the ratio of the two quantities of the same kind (acres in this can be tried in a simpler way with common glass marbles. Lay case) is an abstract number, by which the other quantity, the two thin crips of wood along a smooth surface, like the top of a £95, is multiplied. When we state the question in the second table, and adju.3t their distance so that a marble may just roll way, and talk about multiplying the means and extremes along between them; or, better still, cut a small groove in which together, somo ccnfusion might arise from the idea of multithe marbles may run. One marble may then be laid in the plying 37 acres by £95. The fact to be borne in mind is that groove, and another made to strike it gently. The latter will the rule is merely the expression of the fact that the ratios of come almost to rest, while the other will move. The reason

two pairs of quantities are equal. why it does not come absolutely to rest is, that glass is not 5. The example we have given is what is called a case of

perfectly elastic, and thus the reaction is not direct Proportion--that is to say, if one quantity were increased, quite sufficient to destroy the motion. If seve. the corresponding quantity of the other kind would be increased.

ral marbles be laid so as to touch one another, Thus, if the number of acres were increased, the number of E and one made to strike the end, the same re- pounds they cost would be increased. sults will ensue as with the ivory balls.

If, however, the case be such that, as one of these corre There is one other law relating to impact. sponding quantities be increased, the other is proportionally

It is, that “the angle of incidence is equal to the diminished, the case is one of what is called Inverse Propartiok. Fig. 103. angle of reflection.” The meaning of this will For instance :be clear from the annexed figure.

EXAMPLE.--If 35 men eat a certain quantity of bread in 20 body strike against a surface ac, in the direction D B, it will re. days, how long will it take 50 mon to eat it? bound from it in the direction 'B E, making the same angle with Here, evidently, the more men there are, the less time will the perpendicular BP that B D does. The angle D BF, or that they take to eat the bread; hence, as the number of men at which it strikes A c, is called the angle of incidonco, while increases, the corresponding quantity of the other kind—vi. PB E is the angle of reflection, and the law asserts that these the number of days-decreases. are always equal. As wo pass to optics and other branches of Hence, since 50 men are more than 35 men, the required physics, we shall find further illustrations of this law,

number of days will be fewer than the 20 days which corte

spond to the 35 men. ANSWERS TO EXALIPLES IN LESSON XVII.

In stating the proportion, therefore, in order to make the 1. It will rise a little over 156 feet, and will reach the earth again in ratios equal, if we place the larger of the two terms of one ratio 61 seconds.

in the first place, wo must place the larger of the two terms of 2. The elevation is 51' x 16, which equals 301 16, or 484 feet.

the other ratio in the third place. 3. It will strike tho earth with a velocity of 16).

Thus, placing 50 men in the first place, we must put 20 4. It will take 7 seconds, in the last or which it will fall 208 fcet. days (which, we can see, will be larger than the required 5. 18 * 33, or 528 fect.

answer) in the third place, and then the statement would be 6. It would require 6 seconds, and pass over 576 feet.

correctly made thus :
50

20 days

: required number of days. LESSONS IN ARITHMETIC.-XXXII.

Therefore the required number of days = 20 * 3 days = 14 days.

N.B.-We might reduce the example to a case of Direct RULE OF THREE--SINGLE AND DOUBLE.

Proportion thus, which will, perhaps, explain the above method 1. This is a name given to the application of the principles of more clearly :Simple Proportion to concreto quantities. We have shown

of the bread in one day. (Art. 5, Lesson XX., Vol. I., page 343) that if any three numbers be given, a fourth can always be found such that the four

required number of days numbers shall be proportionals. Hence, if three concreta quantities be given, two of which are of the same kind, and the the number of men, we have

Hence, since the quantity oaten in one day will increase with third of another kind, a fourth quantity of tho same kind as the third can be found such that it wall bear the same ratio

50 : ; to

required number of days; to the third quantity as the first two hear to each other; Therefore required time = 20 ~ ! = 14 days, as before.

D

E

Let any

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6. The last question might also have been solved thus :

GEOMETRICAL PERSPECTIVE.-III. 35 men eat the quantity in 20 days;

BEFORE proceeding farther and deeper into our subject, we wish Therofore 1 man eats al: of the quantity in 20 days; 1

to draw the pupil's attention to an explanation of projection, a Therefore 1 man eats of the quantity in 1 day; 35 x 20

term applied not only to perspective but also to other systems of 50

representation, namely, orthographic and isometric. Our reason Therefore 59 men cat

of the quantity in 1 day ; 20 x 35

for introducing this now, is in order to make it clearly under1

20 x 95 And therefore 50 men would occupy

stood how tho plan of an object is to be treated when we are 50

days in

50 eating the bread,

about to make a perspective drawing of that object, as we very 20 x 35

frequently meet with cases when the plan of the object to be N.B.—To get the time occupied in doing a certain work, represented must bo drawn according to the position which that when the amount done per unit of time (say per day) is given, object presents, whether horizontal or inclined. The plan, as we must evidently divide the whole quantity of work by the wo said in Lesson I., is produced by perpendicular lines drawn amount done in a day. In the case given above, the bread from overy part of an object upon a horizontal plane. Now,

50 being considered the unit, of the bread is caten in 1 day, there can bo no difficulty in drawing a plan when the subject 20 x 35

represented by it is parallel with the ground or horizontal and therefore 50 which is the whole amount eaten divided planą; but it occurs sometimes that it is placed at an angle 20 x 35

with both planes, that is, with the picture-plane and groundby the amount eaten in one day, will be the whole time occupied. plans: therefore in cases of this kind it is necessary to under

stand the first principles of orthographic projection, namely, projec7. Hence we get the following statement of

tion by straight lines upon vertical and horizontal planes. We Simple or Single Rule of Three.

have mentioned above another method of projection, isometric; Write down the ratio of the two quantities which are of the and tren pass it by, as it does not, like orthographic, form any

as the term has been introduced, we will explain its meaning same kind, putting the greater in the first place. Then observing from the nature of the question whether the fourth auxiliary to perspective. The term isometric signifies likequantity required will bo greater or less than the third one

measurement, that is, all the parts of the drawing, both near which is given, place tho greater of the two in the third place and distant, are drawn to one and the same scale, also the plan of the proportion, and multiply the extremes and means

and elevation are combined in one drawing. It is a method together.

much used by architects and engineers when they wish to give

what is generally called a bird's-eye view of a building, etc., EXERCISE 51.- EXAMPLES IN SINGLE RULE OF THREE.

without diminishing the distant parts, as shown in perspective 1. If 16 barrels of flour cost £28, what will 129 cost?

projection. A drawing made isometrically will enable a stranger 2. If 641 sheep cost £485 15s., what will 75 cost ?

to understand the proportions, position, and general character 3. If £11 5s. buy 63 pounds of tea, how many can be bought for of a subject probably better than any other system; henco the

£385 ? 4. A bankrupt pays 6s. 4d. in the pound: what will be received on

reason of its frequent use. a debt of £2,563 10s.?

The extent to which we intend to proceed with orthographic 5. What is £1,160 worth in dollars, allowing 4 dollars 84 cents to projection must be limited to that which relates to, and can a pound, and 100 cents to a dollar ?

assist us in, our present subject, by which we hope to make it 6. If lb. of szuff cost £%, what will 150 lbs. cost?

a valuable auxiliary in our efforts to render the science of per. 7. A man bought of a vessel, and sold { of what he bought spective easy and intelligible.

for £8,240, which was just the cost of it: what was the The differonce between the results of perspective and ortho. whole vessel worth?

graphic projection is caused by the altered position of the oyo 8. If ; of a yard cost i of a crown, what will 3, yards cost? 9. It 10 men build a wall in 7 days, how long would it take 24 only, and from that place is included all that can be seen within

when viewing the object. In perspective the eye is in one place men to build it? 10. If 6 men build a wall in 15 days, how many men would it take

the angle of sight. In orthographic projection the cye is supo just to finish it in 22} days?

posed to be opposite every part at the same time, above the object 11. If } of a ton costs Is. 8 d., what would 47 of a cwt. cost? when the plan is represented, and before it when the cleration is 12. If a twopendy loaf weighs 1 lb. 2 oz. when wheat is 509. a represented; consequently, in perspective, all the visual rays

quarter, what should it weigh when wheat sells for 60s. ? proceeding from the object to the eye converge to one point; 13. If the weight of a cubic inch of distilled water be 253"grains, but in orthographic projection these rays are drawn parallel

and a cubic foot of water weighs 1000 oz. avoirdupois, find the with each other, and perpendicularly to the plane of projection,

number of grains in a pound avoirdupois. 14. If 1 lb. avoirdupois weighs 7,000 grains, and 1 lb. troy weighs whether the plane is horizontal or vertical. To make this clear, 5,760 grains, find how many pounds aroirdupois are equal to

we request the pupil to compare Figs. 5 and 6 of the last 175 lbs. troy.

Lesson with Fig. 8, when he will notice that the characteristic 15. Find the rent of 27a. 3r. 15p. at 21 38. 6d. per acre.

difference between the two systems rests entirely upon tho 16. The price of standard silver being 5s. 6d. per ounce, how many

different treatment of the lines of projection, which, as we have shillings are coined out of a pound troy ?

said, converge in one case, and are parallel in tho other. Fig. 8 17. A bankrupt's assets are £1,500 108., and ho pays 9s. 3}d. in the is to show how a cube is projected orthographically upon vertical

pound: what are his debts ? 18. If standard gold is worth 138.d. per grain, how many sovereigns horizontal,

and horizontal planes of projection. A is the vertical, and B the

c is the cube in space, that is, at a distance from would be coined out of a pound troy of gold ? 19. What is the income of a man who pays 535. 100. tax when it angles of the cube perpendicularly to and meeting the plane B,

both planes of projection. If straight lines are drawn from the is 70. in the pound? 20. Raising the income tax ld. in the round increases my amount and then lines (a, b, c, (?) be drawn to unite them, we shall have of tax by £2 3s. 1d., and the tax I actually pay is £15 3s. 1d. :

i plan of the cube; and as the edges in this case are placed what is the rate of the income-tax?

perpendicularly with the ground, the plan will be a square. 21. A barrel of beer lasts a man and his wife 3 weeks, she drinking Again, if horizontal and parallel lines are drawn from the

half the amount he does : how long would it last 5 such angles of the cube until they meet the vertical plano A, and aro couples ?

then joined by the lines e, f, g, h, we shall produce the elevalion ;

and becauso the horizontal cdges of tho cube are perpendicular KEY TO EXERCISE 50, LESSON XXXI. (Vol. II., page 270).

to the vertical plane of projection, the drawing in this case also

will be a square. Consequently, it will be seen that the drawing e 8. d.

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15. 025.

of tho plan or tho elevation is the same sizo as the object on the 1. 0 10. 45 12 6. 16. 115.

respective plano to which the object is parallel, according to the 2.

5 8.

17. *889 8 14 10.

938,

given scale of that object, as in Figs. 10 and 11. This result 14 11. 132 6 3.

19. 3.786.

makes orthographic projection of much importance for practical 5.

1.
12. 157 15 CM.

20. 15-997. purposes. The working drawings for the guidance of builders 20 18 25. 13.

and mechanists are made by this method. Horizontal lengths 7. 23 11 51. 14. 023.

and breadths are shown both in the plan and elevation, but

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