jars and batteries and of thickness of plates. See Arts. 581-592. "Extract from Dr Williamson's exper. on elect. Eel made in july 1773" p. 1 to 14+ 4 pp. (See Phil. Trans., 1775, p. 94.)

*

In Art. 349, p. 172 of this book, Cavendish uses the expression "when I wrote the second part of this work." It appears from this that he meant it for a book, not a paper to be communicated to the Royal Society. Several portions of this book are contained in the manuscripts, but the order in which they were intended to be placed can be discovered only by help of the figures and diagrams, which are numbered from 1 to 31.

From these it appears that we must begin with No. 4 and No. 5, the Preliminary Propositions and the Appendixt. The Preliminary Propositions refer to the printed paper of 1771. The last proposition in that paper is numbered XXVII., and the first in the MS. is XXIX., so that one proposition appears to be missing, but as there are several drafts, in all of which the first proposition is numbered XXIX., it is probable either that Prop. XXVIII. is not lost, but must be sought for among the enunciations in the second part of the printed paper, or else that Cavendish made a mistake in numbering his propositions.

The Lemmas, however, are numbered consecutively, the last in the printed paper being Lemma XI. and the first in the MS. Lemma XII.

The other mathematical manuscripts are either drafts of these propositions or jottings of calculations not intended for publication.

The paper entitled "Thoughts concerning electricity"§ (No. 18) is placed next. It forms a suitable introduction to the account of the experiments, as it indicates the leading ideas of Cavendish's researches. The paper has no date, but its contents show that it is an earlier form of the theory of electricity, which Cavendish had already abandoned before he wrote the paper of 1771. The pro

This seems to refer to the second part of the paper in the Phil. Trans., 1771, p. 670, or Art. 132 of this edition, and shows that this paper was intended to form the first part of the "Work."

+ Arts. 140 to 174.

Arts. 175 to 194.

§ Arts. 195 to 216.

positions in No. 17 belong to this form of the theory, and are given

in Note 18.

We have next the account of the experiments, the order of which is

The style in which these papers are written leaves no doubt that they were intended to form a book, and to be published. They are given here without any alteration except in the case of a few abbreviations the meaning of which is either obvious or is explained in some other part of the MS. I have also divided them into articles for the sake of more convenient reference. All additions to the MS. are enclosed in square brackets.

After this I have placed the paper on the Torpedo from the Philosophical Transactions for 1776. This, I think, is the whole of the "work" which is extant, but it is by no means a complete account of Cavendish's electrical researches. There are three forms in which Cavendish recorded the results of his experiments:

1st. A Journal containing notes of every observation as it was made, with the particulars of the experiments, and measurements of the apparatus.

2nd. "Results," containing a comparison of the different measures of quantities as recorded in the Journal, and a deduction of the most probable result. See Arts. 647-696.

3rd. I have reproduced the journals for 1771* and 1772† entire, because they form a good example of Cavendish's method of work, and because they contain all the data of some of the most important electrical measurements.

An account of the experiments written for publication.

The journal for 1773‡ is much larger than the others, and gives an account of many interesting and important researches.

Many pages of this journal, however, are filled with the details of the experiments for the comparison of the coated plates which

* Arts. 438 to 465.

+ Arts. 466 to 493.

Arts. 494 to 580.

Cavendish used as standards of capacity. These experiments differ in no respect from those in the former journals, and all the conclusions which Cavendish deduced from them are stated by himself in the "Results." I have therefore thought it best to omit them from the journal, but to retain Cavendish's heading of each experiment and its date when known, and to make the numbers of the omitted articles run on continuously with those retained.

Many of the entries in the journals give the day of the week and of the month, but very few of them give the year. I have therefore ascertained in what years the stated days of the week and month coincided, and have inserted the most probable year within square brackets. It thus appears that the journal entitled "Experiments in 1773" begins with experiments made in October, 1772. Cavendish appears, however, to have got wrong in his reckoning for a good many days together during that month. See Art. 502.

It is somewhat difficult to account for the fact, that though Cavendish had prepared a complete description of his experiments on the charges of bodies, and had even taken the trouble to write out a fair copy, and though all this seems to have been done before 1774, and he continued to make experiments in electricity till 1781, and lived on till 1810, he kept his manuscript by him and never published it. It was not till 1784 that he communicated to the Royal Society those "Experiments on Air," including the production of water and of nitric acid, the absorbing interest of which might perhaps account for some neglect of his electrical writings.

Cavendish cared more for investigation than for publication. He would undertake the most laborious researches in order to clear up a difficulty which no one but himself could appreciate, or was even aware of, and we cannot doubt that the result of his enquiries, when successful, gave him a certain degree of satisfaction. But it did not excite in him that desire to communicate the discovery to others which, in the case of ordinary men of science, generally ensures the publication of their results. How completely these researches of Cavendish remained unknown to other men of science is shown by the external history of electricity.

Viscount Mahon, afterwards Lord Stanhope, a man of great ingenuity and fertility in invention, a pupil of Le Sage of Geneva, and the inventor of the printing press which bears his name, published in 1779 his Principles of Electricity. The theory developed in this book is that

"A positively electrified body surrounded by air will deposit upon all the particles of that Air which shall come successively into "contact with it, a proportional part of its superabundant Electricity, 66 By which means, the Air surrounding that body will also become "positively electrified: that is to say, it will form round that positive body, an electrical atmosphere, which will likewise be positive." (p. 7.) "That the electrical Density of all such Atmospheres decreases, "when the distance from the charged Body is increased." (p. 14.)

66

He then proceeds to determine the law of the density of the electrical atmosphere, as it depends on the distance from the charged body. He assumes that if a cylinder with hemispherical ends is placed in the electrical atmosphere of a charged body, the density of the electricity at any part of the cylinder will depend on the density of the electrical atmosphere in contact with it.

He also shows by experiment that if the cylinder is insulated, and originally without charge, it does not become charged as a whole by being immersed in the electrical atmosphere of a charged body. Hence, when the electricity of the cylinder is disturbed, the whole positive charge on one portion of the surface is numerically equal to the whole negative charge on the other portion.

Now if the density (on the cylinder) were inversely as the distance from the charged body, a transverse section of the cylinder whose distance from the charged body is the geometric mean of the distances of the ends, would divide the charge into two equal parts (both of course of the same kind of electricity), but if the density were inversely as the square of the distance, the distance of the section which would bisect the charge would be the harmonic mean of the distance of the ends. In all this he tacitly confounds the point of bisection of the charge with the neutral point.

He then shows by experiment that the actual position of the neutral point agrees sufficiently well with the harmonic mean, but not with the geometric mean, and from this he concludes (p. 65),

"Consequently, it evidently appears, from what was said above, "that the Density of the Electricity, of the electrical Atmosphere (in "which the said Body A, B was immersed) was in the inverse Ratio of the square of the Distance."

It is evident from this that Lord Mahon was entirely ignorant of everything which Cavendish had done.

About the close of the century Dr Thomas Young, whose acquaintance with all branches of science was as remarkable for its extent as for its profundity, says of this neutral point:

"It was from the situation of this point that Lord Stanhope first "inferred the true law of the electric attractions and repulsions, although "Mr Cavendish had before suggested the same law as the most probable "supposition." (Lecture LIII.)

The same writer, in his "Life of Cavendish," in the Supplement to the Encyclopædia Britannica, gives the following account of the first paper on electricity.

66

"3. An Attempt to explain some of the principal Phenomena of "Electricity by means of an Elastic Fluid. (Phil. Trans. 1771, p. 584.) "Our author's theory of electricity agrees with that which had been "published a few years before by Epinus, but he has entered more minutely into the details of calculation, showing the manner in which "the supposed fluid must be distributed in a variety of cases, and "explaining the phenomena of electrified and charged substances as they are actually observed. There is some degree of unnecessary "complication from the great generality of the determinations: the "law of electric attraction and repulsion not having been at that time "fully ascertained, although Mr Cavendish inclines to the true supposition, of forces varying inversely as the square of the distance: "this deficiency he proposes to supply by future experiments, and leaves "it to more skilful mathematicians to render some other parts of the "theory still more complete. He probably found that the necessity "of the experiments, which he intended to pursue, was afterwards "superseded by those of Lord Stanhope and M. Coulomb; but he "had carried the mathematical investigation somewhat further at a "later period of his life, though he did not publish his papers; an "omission, however, which is the less to be regretted, as M. Poisson, "assisted by all the improvements of modern analysis, has lately treated "the same subject in a very masterly manner. The acknowledged im"perfections, in some parts of Mr Cavendish's demonstrative reasoning,