The words of our next exercise-"The Merry Homes of England"-were written by Mrs. Felicia Dorothea Hemans, one of the most gifted of the English poetesses of the present century. She was born in Liverpool in 1794, and even in early childhood showed an aptitude for versification, which subsequently ripened into poetic talent of a high order. She exchanged her maiden name of Browne for that of Hemans at an early age,

but in consequence of disagreements with her husband, separated from him, and resided first at Wavertree, near Liverpool, and then at Dublin, where she died in 1835. Her writings are remarkable for their purity and refinement; her language, though chaste and simple, being by no means deficient in energy, as may be seen by the following stanzas. The words of this exercise should be sung with spirit.

LESSONS IN

BOOKKEEPING.-X.

SUBSIDIARY BOOKS-CASH BOOK-BILL BOOK.

In the present lesson in Bookkeeping we give our readers a continuation of the example

CASH

of the method of keeping the CASH BOOK, one of the most necessary and importan subsidiary books that are to be found in the offices of all mercantile men, whether ta. wholesale merchants or retail traders. The learner should compare the entries in Cash Book with cash entries in the Memoranda of Transactions. BOOK.

April 1

The next of the subsidary books that demands the learner's notice is the BILL BOOK. The following is the form of this book, which we have adopted, in order to give our students the simplest possible idea of the nature of such a book in business. The first part consists of the Bills Receivable Book, and the second part of the Bills Payable Book. A greater number of columns are frequently introduced into both books for the con

venience of the merchant, and the accuracy which is required in many Bill transactions; but what we have given will be found sufficient for the learner at the outset, as more would tend only to confuse his mind, without imparting any real benefit. This book also includes the entries of all the Bill Transactions from January till June, as laid down in the Memoranda of Transactions given in Lessons VII. and VIII. BILL BOOK.

HYDROSTATICS.-VIII.

FUGAL PUMPS - MACHINES FOR PROPELLING VESSELS

-RIVERS-POWER STORED UP IN RAIN-TIDES-WAVES.

We have in our previous lessons described the construction and mode of operation of those machines for raising water which act mechanically, or by means of atmospheric pressure. These embrace by far the greater portion, and we have now only to look at those which act by centrifugal force.

It may, perhaps, be thought rather unnecessary to explain the construction of so many machines, all of which answer the same purpose; but they each have their special peculiarities, which render one or other of them the more advantageous, according to the special circumstances for which they are required; and as some of them are in use in almost every house and factory, it is surely well to understand their mode of action. It is a very good rule in everything to try and understand the reason, and not to be satisfied with the bare fact that the thing is so. The plumber who understands the principle on which any pump acts will be far more likely to succeed in his business than the one who merely works by routine. The latter is baffled by anything unusual or uncommon, while the other, since he understands the principle on which he is working, soon masters the difficulty. The same rule applies in all other matters.

We have, then, to consider now those pumps which act by centrifugal force. In our lessons on Mechanics we gave an explanation of the action of this force, and we saw then that it is merely a manifestation of the inertia of matter. A stone, for instance, when whirled round by means of a string, tends at every moment to fly off at a tangent, that is, to continue in the line in which it was moving at the instant, instead of being bent round in a curved path.

In the same way, if a tube be filled with water, and swung round rapidly, the water will be thrown out of it.

The apparatus usually employed to illustrate the application of this principle to pumps is represented in Fig. 40.

An upright spindle, c, is fixed so that it can be turned rapidly by means of a multiplying wheel. To this spindle are fixed two tubes, A A, open at each end, but at the top bent outwards and downwards, so that the water which issues from them is received in the ring-shaped trough, B. As these tubes are rapidly rotated with the axis, the water in the upper portion of them is thrown off, by centrifugal force, into the trough. This creates a vacuum in them, which is at once filled from the reservoir into which they open, and thus a continual stream is produced. The amount that could be raised by these pipes is, however, far too small to admit of the machine being practically employed in this form.

Fig. 40.

One of the simplest forms of a centrifugal pump consists of a circular disc fixed on a shaft. Attached to each side of this diso are a number of partitions radiating from the centre; these are made of the same height throughout, so that the whole may revolve between two fixed discs or cheeks which the partitions nearly touch. These cheeks form the ends of the cylinder into which the exitpipe opens, and are cut away at the centre so that the water may enter there; and as the wheel revolves it is thrown off against the sides with such force that it will

rise in the exit-pipe to a considerable height. The size of the wheel is usually about a foot in diameter, that being quite sufficient when it is rotated rapidly, and it is found that when the partitions are made to curve to the right degree nearly three-quarters of the power of the engine may be utilised. If, however, the partitions are made straight, only about a third of the power can be obtained. The great advantages of this pump are the absence of valves, the small space in which it may be made to work, and also the fact that it supplies a continual stream.

Another mode in which the pump is constructed will be under

stood by reference to Fig. 41. The lower is the suction-pipe, and the upper that into which the water is forced. These open into the outer cylinder, which is fixed, and inside this there revolves another cylinder, A A, which has longitudinal slits made in it, in which are inserted the partitions c c. These partitions can slide in and out, but are kept fully out by means of the inner curved surface, except when they approach the division which separates the suction and exit-pipes, when the other curved piece forces them in. After passing this point they are pressed out again.

Now it will easily be seen, by reference to our illustration, that as soon as one of these partitions has passed this point it leaves a vacuum behind it. This is speedily filled by water from the suction-pipe, which is then carried round in the space BB by the next partition. As the cylinder revolves rapidly this water acquires a considerable degree of centrifugal force, which tends to drive it up the exitpipe; the partitions also, as they move onwards, force the water in the same direction. The machine, therefore, acts as a suction and a forcingpump, and has the advantage of maintaining a constant flow of water.

Fig. 41.

With this we conclude our description of machines for raising water, having considered all the most important varieties. We must now turn to the last of the three classes into which we divided all hydraulic machines-namely, those which are designed to propel vessels through the water. Locomotion by water has always been more or less employed, especially since the invention of the mariner's compass; but, till quite recently, the only mode of propelling a vessel was either by human power, employed in rowing or sculling, or else by the force of the winds acting upon sails properly set. The former of these two modes was found to be quite impracticable in the case of large vessels; and though the latter is still very largely used, being inexpensive, there are great disadvantages in its use, arising from the uncertainties of the wind, the ship being often detained, or obliged to tack about frequently when the wind is unfavourable.

Accordingly, very soon after the invention of the steam-engine, an attempt was made to employ it in giving motion to vessels; and though great ridicule was at first thrown on the idea, as happens in the case of most useful inventions, the experiments were successful, and now steamboats are to be seen on almost every river.

The mode in which they are propelled is so familiar that but little need be said here on the subject. There are two plans in common use, the one being by means of paddle-wheels, the other by a screw placed at the stern of the vessel.

The ordinary paddle-wheel is simply a wheel with a number of floats fixed round it, the axle turning on bearings fixed to the boat, and being set in motion by a steam-engine. The principle is exactly the same as that on which a rowing-boat is propelled, the water serving the purpose of a fulcrum; not that it remains absolutely at rest, but the reaction it produces on the surface of the paddles pressing against it is sufficient to propel the vessel with considerable speed. With a well-built vessel an average speed of upwards of twenty miles an hour is easily attained.

There is one circumstance which was at first found to cause a considerable loss of power, and that was that when the paddles were thus fixed to the wheel there was a great loss of power when they entered the water, and when they left it, from the fact of their surfaces not being vertical, so that only a portion of their force was utilised. To remedy this defect, the floats of the best paddle-wheels are now fixed on pivots, and by means of an eccentric are made to move in such a way that while im mersed their surfaces are always vertical, and thus a much larger portion of the power is rendered available. The paddlewheels are usually placed at the side; vessels have, however, been constructed with them at the stern, and these occupy rather less room, and are more available for river navigation.

For a river, or in perfectly smooth weather, a paddle-steamer

is the best, but in rough weather it labours under the great disadvantage that if the vessel be inclined one paddle acts more powerfully than the other, and thus tends to twist the vessel out of her course. In the same way waves interfere with the regularity of the motion. It is also found that there is one certain depth of immersion at which the paddles act best; and if the vessel be loaded so as to sink deeper, or be lighter, in either case there is a considerable loss of power.

The screw is free from these disadvantages, and is therefore frequently used for steamers intended for long sea-voyages. In screw-vessels, instead of a shaft across the vessel, to which the paddle-wheels are fastened, there is one which runs lengthways from the engine-room, and to the end of this the screw is fixed. This consists of two or three large blades twisted somewhat after the plan of a common screw, and as this turns rapidly the water acts the part of a nut, and the vessel is driven forward. Of course the water does not remain fixed, any more than in the case of the paddle-wheel, but, as there, the reaction is sufficient to propel the vessel.

It is an important thing to have the blade inclined at the right angle, and screws have been so contrived that this inclination can be altered at pleasure, but these have not been used in practice. The plan of having two screws side by side is adopted in some large vessels.

A new kind of propeller has recently been tried which acts upon an entirely different principle. The paddle-wheel and screw are entirely dispensed with, and in their stead the engine works some very powerful force-pumps. The water from these is conveyed by large pipes and discharged at the side of the ship, very near to the water-line. Two sets of pipes are fitted up so that the water may be discharged towards the stern or the stem, according to the direction in which it is required to move, and the reaction of the water as it issues serves to propel the vessel. This principle has, at present, only been tried in one or two cases, and therefore it is early to give a definite opinion of its merits.

A model steamboat, which acts on a similar principle, is frequently constructed as a scientific toy. A small brass cylinder is closed at each end, a small hole being drilled in one end, near the circumference, for the escape of the steam. This boiler is filled with water, and placed over a lamp. As soon as the water boils, the steam issues with considerable violence from the small hole, and, striking against the air, causes, by its reaction, the vessel to move rapidly along.

These, then, are the methods of propelling vessels, but there is another question closely connected with this, and that is, What shape should be given to the vessel in order for it to meet with least resistance in passing through the water? This question has attracted much attention from naval architects, it being an important matter to attain the greatest speed in a vessel from a given power of engines. We cannot, however, in the space of these articles, examine the matter. We may roughly state, however, that it is best to let the vessel gradually taper off to the front, and that the shape of the fore-part of a fish, or the beak and head of a bird, approaches somewhat to the form in question. Of course, in considering this, the pressure of the water on the surface has to be resolved along the surface, and at right angles to it.

The same rule applies in the action of rudders. If, as the Tessel is going along, the rudder be inclined to either side, the pressure of the water on it may be resolved into two parts-one acting parallel to it, and therefore producing no effect; the other acting at right angles, and forcing the rudder, and with it the stern of the ship, towards the other side. The effect is thus the same as if the bow were inclined towards the same side as the rudder, and hence the vessel turns that way. The tails of fishes and birds act just like a rudder, and serve to guide them in their flight.

The motions of rivers and the phenomenon of waves and tides are closely connected with the science of hydrostatics, though usually treated of separately. We will, however, just notice the principal facts, leaving it to the student to pursue this subject in books treating more specially of it. A river is a body of fresh water, flowing down an inclined channel towards Now it is evident that the velocity with which it flows will depend, in the first place, on the degree in which its bed is inclined; but the nature of it, whether it is rocky or

the sea.

not, and whether or not it curves about much, will influence the speed to a considerable extent; and according to the speed with which it travels, will be the effects produced on its channel. It is found that a velocity of about one-third of a mile per hour will carry along with it fine sand; from two-thirds of a mile to a mile will carry gravel and small stones; a little greater speed will carry along shingle; while a speed of two miles and upwards will roll along stones almost as large as the fist.

The geological effects of this continual wear are very great, a large amount of silt and sand being carried down and deposited at the bottom of lakes or of the sea.

A continual process of reducing the heights and filling up the hollows on the face of the earth is thus very slowly being carried on, and to the same cause may probably be attributed many of the great geological changes which the surface of the earth has sustained in former ages. Nearly all the solid rocks of the earth were, in fact, deposited under water, and are composed chiefly of débris thus worn down from surrounding parts by the action of water.

If we just consider the amount of rain that falls in any place we shall see what an immense power is stored up in it, which is partly exerted in thus wearing down the surface. A large amount of power is, however, wasted. In fact, some large streams and waterfalls have in them almost inexhaustible stores of power, but little of which is turned to account. It is calculated that the total annual rainfall in England is about two feet, that is, it would cover the surface uniformly to that depth. Now since the weight of a cubic foot of water is about 62 pounds, we see that the weight of rain falling on every square foot of surface is upwards of a hundred-weight; the weight per acre is therefore about 2,400 tons. The mean elevation of England may probably be taken at upwards of 300 feet. The power, therefore, thus stored up in the rain is more than 2,400 x 300 tons per acre. If, therefore, the fears about the exhaustion of coal ever should be realised, we shall find here a copious supply of moving force, and doubtless machines would soon be invented to render a much larger portion of it available.

In tropical regions the rainfall is very much greater than in the latitude of England, in some places reaching as much as 19 or 20 feet in the year. The mean depth over the whole surface of the earth may perhaps be set down at about five feet. The mean elevation, too, of the rest of the world is very much greater than that of England. We see, then, what an immense amount of power is thus produced; and as it is the heat of the sun which turns the water into vapour, and thus raises it, to fall again in the shape of rain, the sun may be said to be the source of all this power.

We have next to consider the motions of the sea, the most important and regular of which are the tides. In every part of the sea, or of a large river, the height of the water is found to vary from hour to hour, attaining its maximum twice in the twenty-four hours, and at intermediate periods being at its lowest level. These alternations always attract attention by their regularity, and it is found that the period of high water is about fifty minutes later on any given day than it was on the preceding day. These motions arise from the attraction of the moon, and will easily be understood by reference to the annexed figure.

Let E represent the earth, and м the position of the moon. Like all other bodies, they attract one another, but since the earth is solid its shape is not at all affected by this attraction. The water, however, is movable, and therefore flows from those parts which are away from the direct influence of the moon towards those which are vertically under it, and thus causes high tide at the latter. Now as the earth revolves on its axis in twenty-four hours, this would cause high water at each part of the earth's surface once in the day, but, as we have seen, there