Of each of these plates of glass Cavendish has given a most minute description, so that each, if it were found, could be identified. Mr Cottrell, of the Royal Institution, has been kind enough to examine the catalogue of apparatus there, which contains Cavendish's Eudiometer and Registering Thermometer. No trace, however, of a set of glass plates could be found. It is possible, however, that if the plates were neatly packed up, their small bulk and their apparent uselessness may have enabled them to survive the periodical overhaulings of some less celebrated repository, and that they may yet gain an honourable place in the museum of historical instruments.

But we need not expect ever to discover a piece of apparatus of still greater historical interest-that by which Cavendish proved that the law of electric repulsion could not differ from that of the inverse square by more than . It consisted of a pair of somewhat rickety wooden frames, to which two hemispheres of pasteboard were fastened by means of sticks of glass. By pulling a string these frames were made to open like a book, showing within the hemispheres the memorable globe of 121 inches diameter, supported on a glass stick as an axis. By pulling the string still more, the hemispheres were drawn quite away from the globe, and a pith ball electrometer was drawn up to the globe to test its "degree of electrification." A machine so bulky, so brittle, and so inelegant was not likely to last long, even in a lumber A facsimile of Cavendish's sketch of it is given at page 104. His own account of the experiment, in Arts. 217-234, is one of the most perfect examples of scientific exposition.

room.

We might also notice the different electrometers, most of them consisting of a pair of cork or pith balls, mounted on straws or on linen threads, and some of them capable of having their weight altered by means of wires run into the straws; but though Cavendish had a wonderful power of making correct observations and getting accurate results with these somewhat clumsy instruments, we must confess that in these, the most vital organs of electric research, Cavendish showed less inventive genius than some of his contemporaries. When Lane and Henly brought out their respective electrometers, Cavendish compared their indications, and by

stating in every case the distance at which Lane's electrometer discharged, he has enabled us to calculate in modern units every degree of electrification that he made use of. What was really needed for Cavendish's experiments was a sensitive electrometer. Cavendish did the best with the electrometers he found in existence, but he did not invent a better one.

It was not till 1785 that Coulomb began to publish the wonderful series of experiments, in which he got such good results with the torsion electrometer, an instrument constructed on the same principle as that with which Cavendish afterwards measured the attraction of gravitation; and it was not till 1787 that Bennett described in the Philosophical Transactions the gold leaf electrometer, by means of which Volta afterwards demonstrated the different electrification of the different metals.

The electrical machine, by Nairne, was one with a glass globe. We should also notice the dividing engine, by Bird, for determining the thickness of the glass plates, and other small distances.

An attendant, "Richard," appears occasionally, to help in turning the electrical machine, or in pulling the strings which made or broke the electrical connexions; and sometimes he is even asked his opinion as to the comparative strength of two electric shocks+. But there is no record of any other person having being admitted into the laboratory during the series of experiments to which we now refer.

The authority of Cavendish in electrical science was of course established by his paper of 1771, and accordingly we find him nominated by the Royal Society as one of a committee appointed in 1772"to consider of a method for securing the powder magazine at Purfleet t."

A powder mill at Brescia having blown up in consequence of being struck by lightning, the Board of Ordnance applied to Mr Benjamin Wilson, F.R.S., who held the contract for the housepainting under the Board§, and who had some reputation as an

Arts. 242, 560, 565.

See Franklin's Works, Vol. v, p. 430, note.

+ Art. 511.

§ He also painted portraits of Franklin and of Gowin Knight, as well as of Garrick in various characters.

electrician, for a method to prevent a like accident to their magazines at Purfleet. Mr Wilson having advised a blunt conductor, and it being understood that Dr Franklin's opinion, formed upon the spot, was for a pointed one, the matter was referred, in 1772, to the Royal Society, and by them as usual to a committee, who after consultation presented a method conformable to Dr Franklin's theory *.

The Committee consisted of Cavendish, Dr, afterwards Sir William Watson; Dr Franklin, Mr J. Robertson (Clerk and Librarian to the Royal Society); Mr Wilson and Mr Delaval.

Dr Franklin read to the Committee a paper which is printed in, his works, Vol. v, p. 435, but is not referred to in the report of the Committee, though the report is entirely in conformity with itt.

The Committee went down to Purfleet and examined all the buildings together, but I cannot trace any evidence that Cavendish did anything to modify the report, and Franklin does not mention him in any part of his writings, as one of the remarkable men with whom he was brought in contact.

The most noteworthy incident of the Committee was the dissent of Mr Wilson, to which Mr Delaval adhered as regards that part of the report which recommended the conductors to be pointed. Mr Wilson followed up his dissent by a paper §, in which he gave his reasons for preferring blunt conductors; but the other four members of the Committee, Messrs Cavendish, Franklin, Watson, and Robertson, having heard and considered these objections, found no reason to change their opinion or vary from their Report ||.

But on the 15th May 1777, the Board House at Purfleet was struck by lightning, and some of the brickwork damaged. This being communicated by the Board of Ordnance to the Royal Society¶, a Committee was appointed to examine the effects of the lightning and to report.

* Phil. Trans., 1772, p. 42.

The report is printed in Franklin's Works, Vol. v, p. 430, and is there stated to be "Drawn up by Benjamin Franklin, August 21, 1772." The paper on the utility of long, pointed rods is stated to have been read on August 27th, 1772.

Ib., p. 48.

§ Ib., p. 49.

Ib., p. 66.

Ib., 1778, p. 232.

The Committee consisted of Mr Henly, Mr Lane, Mr Nairne and Mr Planta, Secretary of the Royal Society. They reported in favour of making a channel all round the parapet and filling it with lead, and connecting this in four places with the main conductor on the roof of the building.

Mr Wilson, however, dissented from this Report, and communicated to the Royal Society an account of a most elaborate and indeed magnificent set of experiments conducted in the Pantheon, in which a cylinder 155 feet long, composed of 120 drums, and connected with a wire 4800 feet long, suspended on silk strings, was electrified, and the discharge made to strike a model of the Board House at Purfleet. The experiments were witnessed by King George III., and seem to have been very brilliant. The picture of the experiment, probably drawn by Mr Wilson, is, as a work of art, considerably above the average of the plates in the Philosophical Transactions.

The subject was referred to a larger Committee, consisting of Sir John Pringle, President of the Royal Society, Dr Watson, Henry Cavendish, W. Henly, Bishop Horsley, T. Lane, Lord Mahon, Edw. Nairne, and Dr Priestley.

They reported* in favour of having an additional number of conductors ten feet high, terminated with pieces of copper eighteen inches long, and as finely tapered and acutely pointed as possible.

"We give these directions," they conclude, "being persuaded, that "elevated rods are preferable to low conductors terminated in rounded "ends, knobs, or balls of metal; and conceiving, that the experiments "and reasons made and alledged to the contrary by Mr Wilson, are "inconclusive."

I have stated this incident at some length because it does not appear to have been noticed by Cavendish's biographers, and because it shows him cooperating with Franklin and others in an electrical investigation undertaken in the interest of the nation.

Cavendish's researches on the electric current have been hitherto very imperfectly known, as they are only alluded to in his celebrated paper on the Torpedo. The private investigations of Cavendish are contained in this volume, but the ex

* Phil. Trans., 1778, p. 313.

ternal events which were more or less connected with them, were as follows:

On July 1, 1773, Mr Walsh communicated to the Royal Society his paper "Of the Electric Property of the Torpedo. In a Letter from John Walsh, Esq., F.R.S., to Benjamin Franklin, Esq., LL.D., F.R.S., Ac. R. Par. Soc. Ext., &c."

The following extracts will indicate the chief points of electrical interest.

"The vigour of the fresh taken Torpedos at the Isle of Ré was not "able to force the torpedinal fluid across the minutest tract of air; "not from one link of a small chain, suspended freely, to another; not "through an almost invisible separation, made by the edge of a pen"knife in a slip of tinfoil pasted on sealing-wax."

"The effect produced by the Torpedo when in air appeared, on many repeated experiments to be about four times as strong as when "in water."

"The Torpedo, on this occasion, dispensed only the distinct instan"taneous stroke, so well known by the name of the electric shock. "That protracted but lighter sensation, that Torpor or Numbness which "he at times induces, and from which he takes his name, was not then "experienced from the animal; but it was imitated with artificial elec"tricity, and shewn to be producible by a quick succession of minute "shocks. This in the Torpedo may perhaps be effected by the suc"cessive discharge of his numerous cylinders, in the nature of a running "fire of musketry; the strong single shock may be his general volley. "In the continued effect, as well as in the instantaneous, his eyes, usually "prominent, are withdrawn into their sockets."

Walsh shows that these phenomena "are in no ways repugnant to the laws of electricity," for "the same quantity of electric matter, according as it is used in a dense or rare state, will produce the different consequences."

"Let me here remark that the sagacity of Mr Cavendish in devising "and his address in executing electrical experiments, led him the first "to experience with artificial electricity, that a shock could be received "from a charge which was unable to force a passage through the least "space of air."

Walsh concludes his letter to Franklin in the following

terms:

"I rejoice in addressing these communications to You. He, who "predicted and shewed that electricity wings the formidable bolt of the "atmosphere, will hear with attention, that in the deep it speeds an "humbler bolt, silent and invisible: He, who analysed the electrified Phial, will hear with pleasure that its laws prevail in animate Phials :

M.

d