9+26; when 3 coincides with division 12, the reading will become 9+, and so on, till finally, when the point 10 coincides with 19 of the scale, the distance 9 will have been increased by 18b, and will become 10, as it should, since, in that case, the 0 point will have been moved a whole space, and will coincide with the division 10 of the limb. Hence, the following rule for reading an instrument which has a vernier.

Read the limb in the direction of the graduation up to the division line next preceding the 0 point of the vernier; this is called the reading on the limb. Look along the vernier till a dividing line is found to coincide with a line of the limb: multiply the number of this first line by the least count of the ver nier: this is the reading on the vernier: the sum of these two readings is the reading of the instrument.

23. In the theodolite described, the limb is divided into half degrees, and 30 spaces on the vernier cover 29 spaces on the limb. Hence, the least count of this instrument is of a half degree or 1'. Fig. 2, Plate 1, exhibits the vernier of the horizontal limb, and Fig. 3 the vernier of the vertical limb.

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TO MEASURE A HORIZONTAL ANGLE WITH THE THEODOLITE.

24. Place the axis of the instrument directly over the point at which the angle is to be measured. This is effected by means of a plumb, suspended from the plate which forms the upper end of the tripod.

Having made the limb truly level, place the 0 of the vernier at 0 or 360° of the limb, and fasten the clampscrew S of the vernier plate. Then, facing in the direc tion between the lines which subtend the angle to be measured, turn the limb with the outer spindle, until the telescope points to the object on the left, very nearly. Clamp the limb with the clamp-screw K, and by means of the tangent screws L and Z, bring the intersection of the spider's lines to coincide exactly with the object.

Having loosened the clamp-screw of the lower tele

same object at which the upper telescope is directed; then tighten the clamp-screw Q. This being done, loosen the clamp-screw S of the vernier plate, and direct the telescope to the other object: the arc passed over by the 0 point of the vernier, is the measure of the angle sought.

The lower telescope having been made fast to the limb, will indicate any change of the position of the limb, should any have taken place; and, as the accuracy of the mea surements depends on the fixedness of the limb, the lower telescope ought to be often examined, and if its position has been altered, the limb must be brought back to its place by the tangent-screw L.

It is not necessary to place the 0 point of the vernier at the 0 point of the limb, previously to commencing the measurement of the angle, but convenient merely; for, whatever be the position of this point on the limb, it is evident that the arc which it passes over is the true measure of the horizontal angle. If, therefore, its place be carefully noted for the first direction, and also for the second, the difference of these two readings will be the true angle, unless the 0 point of the vernier shall have passed the 0 point of the limb, in which case the greater reading must be subtracted from 360°, and the remainder added to the less.

TO MEASURE A VERTICAL ANGLE.

25. We shall first explain the method of determining the index error. Having levelled the horizontal limb, direct the telescope to some distinctly marked object as the top of a chimney, and read the instrument. Reverse the telescope in the Y's, and turn the vernier plate 180°, and having directed the telescope to the same object, again read the instrument. If the two readings are the same, the limb is adjusted; that is, the 0 of the limb coincides with the 0 of its vernier, when the axis of the telescope is parallel to the horizontal limb.

When the reading found with the eye end of the telescope nearest the vernier, is greater than that obtained in

which is equal to a mean of the readings, may be obtained by subtracting half their difference from the first reading. If the first reading is less than the second, the half difference must be added to the first. Hence,

To find the index error, take the reading of the limb when the telescope is directed to a fixed object, first with the eye end of the telescope nearest the vernier, and then with the telescope and vernier plate both reversed. Take half the difference of

these readings, and affect it with a minus sign if the first is greater, or a plus sign if the second is the greater; this is equal to the index error.

Let the operation be repeated several times, using dif ferent objects, and a mean of the errors will be more correct than the result of a single observation.

26. Having determined the index error, let the axis of the telescope be directed to any point either above or be low the plane of the limb, and read the arc indicated by the 0 of the vernier. To the arc so read apply the proper correction, if any, and the result will be the true angle of elevation or depression.

The angle of elevation may be more correctly found by taking the elevation of the object, and repeating the observation with the telescope and vernier plate reversed, and then taking a mean of the readings for the angle required.

MEASUREMENTS WITH THE TAPE OR CHAIN ONLY.

27. It often happens that instruments for the measur ment of angles cannot be easily obtained; we must then rely entirely on the tape or chain.

We now propose to explain the best methods of determining distances, without the aid of instruments for the measurement of horizontal or vertical angles.

I. To trace, on the ground, the direction of a right line, that shall be perpendicular at a given point, to a given right line.

FIRST METHOD.

point. Measure from A, on the line BC, two equal distances AB, AC, one on each side of the point A. Take a portion of the chain or tape, greater than AB, and

place one extremity at B, and with the other trace the arc of a circle on the ground. Then remove the end which was at B, to C, and trace a second arc intersecting the former at D. The straight line drawn through D and A will be perpendicular to BC at A.

SECOND METHOD.

B

29. Having made AB= AC, take any portion of the tape or chain considerably greater than the distance between B and C. Mark the middle point of it, and fasten its two extremities, the one at B and the other at C. Then, taking the chain by the middle point, stretch it tightly on either side of BC, and place a staff at D or E: DAE will be the perpendicular required.

E

THIRD METHOD.

30. Let AB be the given line, and the point at which the perpendicular is to be drawn. From the point measure a distance CA equal to 8. With C as a centre, and a radius equal to 6, describe an arc on either side of AB: then,

A

-B

with A as a centre, and a radius equal to 10, describe a second arc intersecting at E, the one before described: then draw the line EC, and it will be perpendicular to AB at C.

REMARK. Any three lines, having the ratio of 6, 8, and 10, form a right-angled triangle, of which the side corre

FOURTH METHOD.

31. Let AD be the given right line, and D the point at which the perpendicular is to be drawn. Take any distance on the tape or chain, and place one extremity at D, and fasten the other at some point, as E, between

the two lines which are to form the right angle. Place a staff at E. Then, having stationed a person at D, remove that extremity of the chain and carry it round until it ranges on the line DA at A. Place a staff at A: then remove the end of the chain at A, and carry it round until it falls on the line AE at F. Then place a staff at F; ADF will be a right angle, being an angle in a semicircle.

32. There is a very simple instrument, used exclusively in laying off right angles on the ground, which is called the

SURVEYING CROSS.

Pl. 2, Fig. 1. This instrument consists of two bars, AB and CD, permanently fixed at right angles to each other, and firmly attached at E to a pointed staff, which serves as a support. Four sights are screwed firmly to the bars, by means of the screws a, b, c, and d.

As the only use of this instrument is to lay off right angles, it is of the first importance that the lines of sight be truly at right angles. To ascertain if they are so, let the bar AB be turned until its sights mark some distinct object; then look through the other sights, and place a staff on the line which they indicate: let the cross be then turned until the sights of the bar AB come to the same line: if the other sights are directed to the first object, the lines of sight are exactly at right angles.

The sights being at right angles, if one of them be turned in the direction of a given line, the other will mark the direction of a line perpendicular to it, at the point where the instrument is placed.

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