The speed of electricity in passing through a conductor mainly depends upon the inductive capacity of the surrounding substances, and, except for technical or special purposes, there is little use in measuring velocities which in some cases are one hundred times as great as in other cases. But the maximum speed of electric conduction is probably a constant quantity of great scientific importance, and according to Prof. Clerk Maxwell's determination in 1868 is 174,800 miles per second, or little less than that of light. The true boiling point of water is a point on which all practical thermometry depends, and it is highly important to determine that point in relation to the absolute thermometric scale. But when water free from air and impurity is heated there seems to be no definite limit to the temperature it may reach, a temperature of 356° Fahr. having been actually observed. Such temperatures, therefore, do not require very accurate measurement. All meteorological measurements depending on the accidental condition of the sky are of infinitely less importance than physical measurements in which such accidental conditions do not intervene. Many profound investigations depend upon our knowledge of the radiant energy continually poured upon the earth by the sun; but this must be measured when the sky is perfectly clear, and the absorption of the atmosphere at its minimum. The slightest interference of cloud destroys the value of such a measurement, except for meteorological purposes, which are of vastly less generality and importance. It is seldom useful, again, to measure such a quantity as the height of a snow-covered mountain within a foot, when the thickness of the snow alone may cause it to vary 25 feet or more, when in short the height itself is indefinite to that

Maintenance of Similar Conditions,

Our ultimate object in induction must be to obtain the complete relation between the conditions and the effect, but this relation will generally be so complex that we can only attack it in detail. We must, as far as possible, confine the variation to one condition at a time, and establish a separate relation between each condition and the effect. This will be at any rate the first step in approximating to the complete law, and it will be a subsequent question how far the simultaneous variation of several conditions. modifies their separate actions. In many of the most important experiments, indeed, it is only one condition which we wish to study, and the others are merely interfering forces which we would gladly avoid if possible. One of the conditions of the motion of a pendulum is the resistance of the air, or other medium in which it swings; but when Newton was desirous of proving the equal gravitation of all substances, he had no interest in so entirely different a force as the effect of the air. His object was then to observe a single force only, and so it is in a great many other experiments. Accordingly one of the most important methods of investigation consiste in maintaining all the conditions of like magnitude except that which is to be studied. As that admirable experimental philosopher, Gilbert, expressed it, "There is always need of similar preparation, of similar figure, and of equal magnitude, for in dissimilar and vegal execumstances the experiment is doubwhp?

In Newson's decisive ezzaninana similar onditions were provided for, wjsn tra vanad zingidity which characterizes the I ́erest wi The pendulume of which the oscillanions were compered semained of ezarny argual toxes of woods Bangrit goes onwoude, and fled won diferent sibe

stances, so that the total weights should be exactly equal and the centres of oscillation at the same distance from the points of suspension. Hence the resistance of the air became approximately a matter of indifference; for the outward size and shape of the pendulums being exactly the same, the absolute force of resistance would be the same, so long as the pendulums vibrated with equal velocity; and the weights being equal the force would diminish the velocity in like degree. Hence if any inequality were observed in the vibrations of the two pendulums, it must arise from the only circumstance which was different, namely the chemical character of the matter within the boxes. No inequality being observed, the chemical nature of substances can have no appreciable influence upon the force of gravitations.

A beautiful experiment was devised by Dr. Joule for the purpose of showing that the gain or loss of heat by a gas is connected, not with the mere change of its volume and density, but with the energy received or given out by the gas. Two strong vessels, connected by a tube and stopcock, were surrounded entirely with water after the air had been exhausted from one vessel and condensed in the other to the extent of twenty atmospheres. The whole apparatus having been brought to a uniform temperature by agitating the water, and the temperature having been exactly observed, the stop-cock was opened, so that the air at once expanded and filled the two vessels uniformly. The temperature of the water being again noted was found to be almost entirely unchanged. The experiment was then repeated in an exactly similar manner, except that the strong vessels were placed in separate portions of water. It was then discovered that cold was produced in the vessel from which the air rushed, and an almost exactly equal quantity of heat appeared in that to which 'Principia,' bk. III. Prop. vi.

it was conducted. Thus Dr. Joule clearly proved that rarefaction produces as much heat as cold, and that only when there is a disappearance of mechanical energy will there be production of heath. What we have to notice, however, is not so much the result of the experiment, as the admirably simple manner in which a single change in the apparatus, the separation of the portions of water surrounding the strong air vessels, is made to give indications of the utmost significance.

Collective Experiments.

There is an interesting class of experiments which enable us to observe an indefinite number of quantitative results in one act. Generally speaking, each experiment yields us but one number, and before we can approach the real processes of reasoning we must laboriously repeat measurement after measurement, until we can lay out a pretty complete curve of the variation of one quantity as depending on another. Now we can sometimes abbreviate this labour, by making one quantity vary in different parts of the same apparatus through every required amount. Thus in observing the height to which water rises by the capillary attraction of a glass vessel, we may take a series of glass tubes of different bore, and measure the height through which it rises in each. But if we take two glass plates, and place them vertically in water, so as to be in contact at one vertical side, and slightly separated at the other side, the interval between the plates varies through every intermediate width, and the water rises to a corresponding height, producing at its upper surface a hyperbolic curve.

The absorption of light in passing through a coloured liquid may be beautifully shown by enclosing the liquid

h Philosophical Magazine,' 3rd Series, vol. xxvi. p. 375.

in a wedge-shaped glass, so that we have at a single glance an infinite variety of thicknesses in view. As Newton himself remarked, a red liquid viewed in this manner is found to have a pale yellow colour at the thinnest part, and it passes through orange into red, which gradually becomes of a deeper and darker tinti. The effect may be noticed even in a common conical wineglass. The prismatic analysis of light from such a wedgeshaped vessel discloses the reason, by exhibiting the progressive absorption of different rays of the spectrum as investigated by Dr. J. H. Gladstonek.

A moving body may sometimes be made to mark out its own course, like a shooting star which leaves a tail behind it. Thus an inclined jet of water exhibits in the clearest manner the parabolic path of a projectile. In Wheatstone's Kaleidophone the curves produced by the combination of vibrations of different ratios are shown by placing bright reflective buttons on the tops of wires of various forms. The motions are performed so quickly that the eye receives the impression of the path as a complete whole, just as a burning stick whirled round produces a continuous circle. The laws of electric induction are beautifully shown when iron filings are brought under the influence of a magnet, and fall into curves corresponding to what Faraday called the Lines of Magnetic Force. When Faraday tried to define what he meant by his lines of force, he was obliged to refer to the filings. magnetic curves,' he says, 'I mean lines of magnetic forces which would be depicted by iron filings.' Robison had previously produced similar curves by the action of frictional electricitym, and from a mathematical investiga

i Opticks,' 3rd edit. p. 159.

Watts, 'Dictionary of Chemistry,' vol. iii. p. 637.

- Faraday's Life,' by Bence Jones, vol. ii. p. 5.

Watts Dictionary of Chemistry,' vol. ii. pp. 402, 403.

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