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tion; the same is the case with the current from D; thus both currents tend to prodnce motion in the direction of the arrows. (Fig. 9.)

But Water-wheels afford a better means of employing currents of water as a power to drive machinery. Vertical water-wheels are of three kinds-. overshot, breast, and undershot. Fig. 10 shows an overshot wheel. It is used when a fall can be obtained at least equal to the height of the wheel; on it the water acts by its weight, and the buckets are made of such a shape as to retain the water as long as it is of any effect in turning the wheel.

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When such a fall cannot be commanded, we use a breast wheel (Fig. 11). The water acts on it both by its momentum and weight, momentum meaning the force that a quantity of matter possessing some velocity can exert on another body; the momentum of a body is measured by the product of its weight and velocity.

The undershot wheel is used when there is scarcely any fall. It is sometimes turned by tidal streams, and then works in both directions; in this case the water acts only by its momentum. When an undershot wheel is driven by a current which runs always in the same direction there is a decided advantage in having the float-boards a little inclined to the advancing stream, for then the water rolls up the floats and acts by its weight as well as by its momentum. An undershot wheel might be used where a fall could be obtained that would be sufficient for an overshot; but it appears from the experiments of Smeaton that the dimensions, quantity of water, and height of the fall being the same, an overshot will produce double the effect of an undershot wheel. (Fig. 12.)

Various contrivances have been employed in the course of time for raising water. An early and ingenious method was by the screw of Archimedes, invented by that celebrated philosopher about 200 years before Christ. Constructed as in Fig. 13, it consists of a flexible tube, wound round a cylinder in the form of a screw; the lower end of the screw dips into the water to be raised. As each part of the screw changes from a lower to a higher position during a revolution, the water within it falls backwards, and at the same time is raised by the lower surface of the interior; thus, it gradually ascends from A, and is finally ejected at B.

We had occasion to mention before that when the flow of water in a pipe is suddenly stopped, a great lateral pressure is exerted: this is the principle of the HYDRaUL1C Ram (Fig. 14), an instrument invented by Montgolfier, in 1796, by which water can be raised to a height much above its natural level. Through the inclined pipe, C, leading from a reservoir, the water flows; but, on reaching the orifice at D, it has acquired a force sufficient to raise the valve, and thus close the orifice; the flow being thus suddenly stopped, a lateral pressure is exerted so great as to lift the valve E; the water then flows into the chamber B; this condenses the air in B, which, from its elastic force, soon

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closes the valve E again and forces the water up the pipe F. The liquid in A is now quiescent, but in that state is unable to support the heavy valve D, which therefore falls, and allows the water to escape from the orifice; soon such a force is acquired by the water as is sufficient to again close the valve d. The above process is repeated, and thus the action is continued. This engine can only be employed with advantage when there is an abundant supply of water, for more flows out of the orifice at D and is lost than is actually raised through the pipe F.

All other methods for raising water are inferior to the simple and beautiful contrivance of the common pump and force-pump, but the principle of their action belongs to pneumatics.

ELECTRICITY.

The study of electricity has been usually divided into two parts: static, or frictional electricity, and dynamic, or current electricity.

The great advance that has been made in the science has been within, comparatively speaking, very few years: in its early stages the distance between discoveries was very great. Of the earliest known date of the oDservation of electrical phenomena, we find the first to be B.C. 600, when Thales of Miletus, the founder of the Ionic philosophy and a Greek philosopher of celebrity, noticed the remarkable properties of amber excited by friction, which had the peculiar effect of attracting to it straws and pieces of light material. So struck was he with this peculiarity, that he imagined the amber to possess a species of animation.

The next electrical phenomenon was observed by Theophrastus, about B.C. 321, who observed a similar property in a hard stone called "lyncnrium" now supposed to be tourmaline, which not only possessed the power of attracting, similar to amber, light pieces of straw, but even small pieces of metal.

Amongst other ancient philosophers who have made mention of these facts in their writings, I will only refer to Pliny, a.d. 70, who, in speaking of amber, says, "attrilu digitorum accepta vi caloris attrahunt in se paleas etfolia arida ut magnes lapisferrurn."

The observations of the above facts have been handed down to us by writers, a few of whom I have mentioned; but the observations are simply recorded as facts: cause and effect are in no way entered into,and conjecture and reasoning relative to such extraordinary effects do not appear to have entered into their writings.

To the peculiar effect of amber excited by friction do we fall back for the name given to the science. Electricity is derived from "amber," in Greek ■tjXeKTpov (electron), and in Latin electrum, hence "electricity."

If you will take a piece of amber and rub it with some dry woollen stuff, you will find the peculiar attractive properties noticed above: it will readily attract small pieces of paper, &c Amber so excited is called "electrified."

From the period mentioned above but little observations appear to have been taken until Dr. Gilbert, in the sixteenth century, instituted a series of experi ments upon electrical attraction. He found that amber was not the only sub* stance possessing that peculiarity, but that it belonged to many bodies, among which were glass, sulphur, sealing-wax, and resin. It was subsequently observed that the attractive power was increased upon warming the substance.

The principal discovery that added to the advancement of the science was that of Otto Guericke, of Magdeburg, who was the first to invent an electrical machine. A globe of sulphur was mounted on an axis, and caused to revolve rapidly against a rubber. By this means a greater quantity of electricity was obtained, and its principal features experimented upon and recorded.

Sir Isaac Newton contributed several important discoveries, and in 1710 a globe of glass was substituted by Hawkesbee (some say Newton) for the globe of sulphur; this he mounted in a wooden frame, and it is, indeed, very similar to those in use at the present time.

The following experiments will enable you to observe some of the peculiar effects of electrified bodies; but first provide yourself with these articles: a tube of stout glass about 1J ft. long, an ebonite ruler or paper-cutter, a stick of sealing-wax, and two pairs of pith balls attached to some silk thread.

1st Experiment.—AVarm the glass tube, and with a dry silk handkerchief excite it by rubbing it several times; then it will become electrified and capable of exercising its attractive powers. On a small stand or convenient support hang one of the pith balls (a stand somewhat similar to Fig. 1 * is most convenient); present the glass towards it: the pith ball will immediately fly towards the glass and cling to it for a short space of time; it will then drop away and remain at some distance from it. If you follow it with the glass, you will see it try to avoid the glass. In the former case the glass exhibits electrical attraction, and in the latter, electrical repulsion.

* / glass rod bent to the required form and attached by sealing-wax to a small wood base.

Next excite the sealing-wax or ebonite with some warm flannel; similar re* suits will be observed, the two bodies showing attraction and repulsion.

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2nd Experiment.—Excite the glass and then the sealing-wax, and present the glass to the pith ball; it will, as before, be immediately attracted and then repelled. Now present the sealing-wax: it will immediately attract the pith ball. Reverse the experiment, and first present the sealing-wax: the ball will be, as before, attracted and repelled; but immediately on the presentation of the excited glass it will be attracted.

From this it appears that these bodies, which both attract light substances, exhibit a different kind of force; they are consequently called, those exercising the same attractive property as glass, vitreous electricity; and those similar to the wax, resinous electricity.

The fundamental rule is that bodies charged with the same kind of electricity repel each other; charged with opposite kinds of electricity they attract each other.

Suspend the two pith balls from the same support, Fig. 2; charge them with the glass—the pith balls will repel each other; touch them in order to discharge them, as, until bodies which have been charged have touched the earth or some substance in connection with it, they will remain charged and cause repulsion. Next charge them with the excited wax or ebonite; they will again repel one another, showing that they are charged with the same electricity exercising mutual repulsion.

Fig. 3.

Suspend the two balls from two points near each other; charge one with

the glass, and the other with the wax: on bringing the balls near they will be found to be violently attracted to eac other.

That the terms vitreous and resinous are not without some objections, the following experiments will prove:

Excite the sealing-wax with the woollen cloth; present it to the pith ball: it will first attract and then repel the pith ball. Now present the woollen cloth — the pith ball is immediately attracted, showing that the cloth is vitreously or excited with electricity of an opposite nature.

Perform a similar experiment with the glass tube: it will be found that the woollen cloth will attract the ball that the glass repelled; this would show that the cloth was rcsinously excited.

Next, if the glass be rubbed first with a woollen cloth and then with the fur of a cat, it will be found to be excited in the first place with vitreous, and in the second with resinous electricity. Both glass and woollen cloth exhibit, therefore, both kinds of electricity.

These terms have, therefore, been considered objectionable, and positive and negative are now generally substituted for them; thus glass excited with a woollen or silk cloth is said to exhibit positive electricity; sealing-wax exhibits negative electricity when similarly excited.

Excite the sealing-wax or ebonite as before; but, instead of having pith balls suspended on silk threads, let them be hung on thin wire or cotton thread; present the ebonite—the pith ball will be found to be permanently attracted, and will remain so as long as there is any electric excitement in the ebonite. Under these circumstances, it shows that it cannot retain a permanent charge, and that the electricity communicated to it is conducted away by the cotton. A body, therefore, will not retain a charge unless insulated.

Various bodies conduct or insulate differently: the two terms as usually applied mean the opposite condition. Faraday says that conduction and insulation are only extreme degrees of one common condition. A body that insulates well must of necessity be a bad conductor; on the other hand, a body that conducts well must be a bad insulator.

Of conductors, the various metals are the best, then bodies in a fluid state; of insulators, gums—such as India-rubber, gutta-percha—bodies like glass, amber, wax, jet, &c, are the best; but between these extremes, the one condition gradually merges into the other.

In any experiments that maybe made with frictional electricity, it is necessary that everything should be kept dry, as insulators, when moist or damp, become, practically speaking, conductors.

Amongst the instruments used for detecting electricity are some by which the electricity can be measured: these are called Electrometers; those for

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