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oy a Grove's element, and the distance in the spark-micrometer F was made 2 millims., as neither larger nor smaller distances gave such good results.

Under these circumstances it was sufficient, in order to produce a spark, if one wire D was even only 1 decimeter longer than the other. When, on the contrary, they were of the same length, a spark never appeared. Yet it can be instantaneously evoked if, by touching one of the wires with the knob of a Leyden jar, the symmetry of the two current-paths is disturbed.

In these experiments also the material and thickness of the wire exerted not the smallest influence. Whether I used a silvered copper wire of 0.06 millim. diameter, or an iron wire of 0.23, or a copper wire of 0.8 millim. diameter, the spark never appeared when both wires were of the same length.

Hence the velocity of the propagation of electricity is the same for all stretched* wires.

Yet in the form above described, the experiment is not very striking, as we can only work with very small distances in the accessory micrometer f. I endeavoured therefore to alter it in such a manner that it would be visible to a whole audience.

Experiments with small Geissler's tubes have led to no decisive result. On the contrary, with lengths of some metres at least, the retardation may be very beautifully shown in the following

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If a discharge (negative) (best of all, of a Ruhmkorff's apparatus) be divided, as above, just behind the spark-micrometer into two branches, and if one of them be connected with the coating of the perfectly insulated test-plate, while the other is led by the conductor A to the upper uncoated surface, a positive, a negative, or no figure at all can be made to appear, according as the upper branch is larger, smaller, or as long as the lower one. Indeed the experiments must succeed one another in a definite order if they are intended to support the opinion that they owe their origin to differences in time. For if we remember that it is immaterial whether positive electricity be imparted to the plate

* Spirally-coiled wires will, it may be presumed, give a different result. Phil. Mag. S. 4. Vol. 40. No. 264. July 1870.

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or negative be extracted, it will be understood that a positive charge produces a positive figure when the electricity reaches the point of the conductor before it reaches the coating-that is, when D, is shorter than D. If, on the other hand, the discharge reaches the coating first, the conductor will be traversed by the induced electricity in the opposite direction; and hence a negative figure must be formed upon the glass surface when D2 is shorter than D1. In the course of the motion this induction discharge must meet, in the wire D,, the electricity coming direct from F, and thereby a compound character will be imparted to the figure.

Between these two arrangements with entirely opposite results there must obviously be some in which no figures are formed, as there is no reason why one should be formed in preference to the other. This must be the case when the electricities from both sides arrive simultaneously--that is, when D, and D, are of the same length*.

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The experiments completely fulfilled these theoretical anticipations. With either kind of electricity, figures of both kinds are obtained when the lengths of the wires are rightly chosen.

To many a one who makes the experiment under not quite favourable circumstances this statement may appear incorrect, apart from the case in which, owing to perfect equality of the two branches, no figures at all are formed; for it may occur that the whole of the figures seem at first sight positive, whatever be the circumstances and whatever be the kind of electricity worked with.

The cause lies simply in the circumstance that the compound negative figures belong in this case to that group which have already a strongly positive character, and even, at first, can scarcely be recognized as negative; but the considerable difference in magnitude which occurs after a change of poles is sufficient at once to remove any doubt as to the true nature of the figures, and to prove the agreement of the experiments with theoretical anticipations.

To sum up, the following results were obtained :

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1. When an electrical discharge, after traversing a spark-interval, is offered two paths to the earth (a short one, and a long one interrupted by a test-plate), with small striking-distances the discharge is divided. With greater distances the electricity takes only the shorter path, and even carries with it electricity of the same kind from the other branch.

2. If electrical waves be sent into a wire insulated at the end,

* There may probably in this case be a small difference in favour of the upper wire, since the electricity coming from below has to spread over 'the entire coating.

they will be reflected at that end. The phenomena which accompany this process in alternating discharges appear to owe their origin to the interference of the entering and reflected waves.

3. An electrical discharge travels with equal rapidity in wires of equal length, without reference to the materials of which these wires are made.

VIII. On the Interchangeability of Heat and Mechanical Action. By the Rev. J. M. HEATH*,

THE

HE doctrine of the equivalence of heat and mechanical action, and that of the conservation of energy, are the expression of one and the same thing to those who believe that heat is motion, and therefore itself a form of force or energy. They both alike express this--that when the action of force, continued through a space, results in motion or heat, or vice versá, there has been a true conversion, a change of one form into another, and that when no motion results there is no conversion. The selfsame expression (fds expresses indifferently the pressure accumulated at any point in a fluid mass by the action of all the particles situated upon a given line upon it, or the accumulation of the same force upon a single particle which should move through the line for which the integral is taken. But the results are not identical. The forces which accelerated the moving particle have done their work and are extinguished; they now exist only in the form of the motion they have created. But the corresponding fluid pressures have done no work (if the creation of motion is work), and have never become any thing else than the pressures they were at first.

If we bear this in mind, the great problem of the science of thermodynamics (how much out of a given gross amount of force (P) applied as a load to the piston of a gas-chamber will generate its mechanical equivalent in heat or motion) becomes one of extreme simplicity. The force P divides itself into two parts, p the pressure of the gas below employed in neutralizing the resistance opposed by the gas to motion, and P―p the remainder, which is wholly effective in producing motion. The separate functions which these two portions of the force respectively discharge are given by the two equations

=

S (P−p)dv= ±Σmv2+C and S (p—p)dv= const.,

where v is the volume of the gas. And it appears very obvious that the first of these is the answer to the question, How much of the whole force P is converted into motion or heat?

* Communicated by the Author.

But this conclusion is now generally ignored, or rather set aside, in favour of a very different one, founded, as I am forced to think, upon a misconception of the very important elementary question in kinetics, How is a weight lifted up? The quantity of heat gained or lost, we are told, in the supposed case depends upon the work expended upon its generation, or upon that done by its destruction. In the first case, if the piston descends through the space dv, the work expended is SPdv or PSv. the second case, if the gas drives the piston up before it, the

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work done is pdv. In which it may be observed, en pas

sant, that the constancy of P affects the form of the result very materially when it descends and causes condensation, but has no influence upon it when it is raised. But the far more serious objection to the doctrine is, that weight raised or resistance overcome is precisely that kind of work done by a force in which no conversion into heat or motion takes place at all; and it excludes the only case in which such conversion does take place, which, as we have shown, is that in which force acting upon matter free to move, itself passes into motion. From the second of the above equations, (p—p) dv=c, we see that so long as the forces above and below the piston remain equal to each other no vis viva is generated. The piston may rise or it may descend, but the motion will not be due to either of the antagonistic forces, whose function it is to reduce each other to nullity. The office of the elasticity of gas in raising a superincumbent weight is simply and exclusively that of giving it statical support at every point of the rise. The force P-p generates acceleration, the forces p-p make unaccelerated rise or fall possible: and it is the singular infelicity of the modern doctrine that heat is created by the expenditure of work, that the definition of the work expended does not include the case in which alone motion or heat is created, and does include only the cases where no motion or heat is created. I do not think I can more distinctly contradict every part of the received doctrine on this subject than by stating simply what appears to me to be unquestionable, as truth-that no force employed in equilibrating resistances ever becomes converted into heat, and that no heat is ever generated except by forces acting on bodies verging on the state of motion, and offering no resistance to the action of the forces.

Milland, June 21, 1870.

IX. Proceedings of Learned Societies.

ROYAL SOCIETY.

[Continued from vol. xxxix. p. 462.]

March 10, 1870.-Warren De La Ru, Esq., Vice-President, in the Chair.

THE following communication was read: II. The Positions and

Areas of the Spots observed at Kew during the years 1864–66, also the Spotted Area of the Sun's visible disk from the commencement of 1832 up to May 1868." By Warren De La Rue, Esq., Ph.D., F.R.S., F.R.A.S., Balfour Stewart, Esq., LL.D., F.R.S., F.R.A.S., &c., and Benjamin Loewy, Esq., F.R.A.S.

The paper commences with a continuation for the years 1864-66 of Tables II. and III. of a previous paper by the same authors; it then proceeds to a discussion of the value of the pictures of the sun made by Hofrath Schwabe, which had been placed at the disposal of the authors; and the result is that these pictures, when compared with simultaneous pictures taken by Carrington and by the Kew heliograph, are found to be of great trustworthiness. From 1832 to 1854 the pictures discussed are those of Schwabe, who was the only observer between these dates; then follows the series taken by Carrington, and lastly the Kew series, which began in 1862.

A list is given of the values of the sun's spotted area for every fortnight, from the beginning of 1832 up to May 1868, and also a list of three-monthly values of the same, each three-monthly value being the mean of the three fortnightly values which precede and of the three which follow it. These three-monthly values are also given for every fortnight.

A plate is appended to the paper, in which a curve is laid down representing the progress of solar disturbance as derived from the three-monthly values; and another curve is derived from this by a simple process of equalization, representing the progress of the tenyearly period. The values of the latter curve, corresponding to every fortnight, are also tabulated. From this Table are derived the following epochs of maxima and minima of the longer period :

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Dec. 21, 1836.

Nov. 14, 1847.

Sept. 7, 1859.

This exhibits a variability in the length of the whole period.

Thus we have between 1st and 2nd minimum..

9.81 years.

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12.58
10.81

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Mean of all the periods........ 11:07 years.

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