P. S. 14th June, 1879.

Just before sending this sheet to press I have received from Mr Robert H. Scott, F.R.S., a small packet marked "Cavendish Papers," which had been sent to the Meteorological Office by Sir Edward Sabine.

These papers relate entirely to magnetism, and do not fall within the scope of this volume, though they may supply important materials for the magnetic history of the earth, and are in all respects excellent specimens of Cavendish's scientific procedure.

I shall therefore only mention a few particulars in which these papers throw some additional light on Cavendish's life and work.

The descriptions of Cavendish by Cuvier, Young, Thomson and Wilson agree in representing him as living in London, and regularly attending the meetings of the Royal Society, but in other respects leading an isolated life, very much detached from the interests, whether social or scientific, of other men.

It has also been hinted that Lord Charles Cavendish, who, as we have already seen, was himself addicted to scientific pursuits, did not entirely approve of his son's devotion to science, or at least, for some reason or other, restricted him in the means of carrying on his work.

In these manuscripts, however, we have the details of a laborious series of observations undertaken to determine the errors of the variation compass and the dipping needle belonging to the Royal Society, and on Sept. 16, 1773, we find "Observations of needle in Garden by Father and Self," and a "Comparison of Society's compass in house and in soc[iety's] garden with Father's compass in room."

It appears, therefore, that Lord Charles Cavendish not only placed his instruments at his son's disposal, but made observations of the compass in concert with him, and that these observations were undertaken in order to make the instruments belonging to the Royal Society more available for accurate measurements. In the same Journal there are also " Measures taken for setting Dr Knight's magnets so that their poles shall be equidistant from var[iation] comp[ass] and dipp[ing] need[le] in 1775." The results of this enquiry are briefly stated by Cavendish in his paper on the Instruments belonging to the Royal Society in the "Philosophical

Transactions" for 1776. In the same volume there is an account of Dr Knight's great Magazines of magnets by Dr Fothergill.

A considerable portion of the MS. is taken up with "Directions for using the Dipping Needle," written out at greater or less length (probably according to the scientific capacity of the recipient) "for Captain Pickersgill," "for Captain Bayley," "for Dalrymple" [Hydrographer to the Honourable East India Company] &c.

There is also a treatise of 26 pages "On the different forms of construction of dipping needles."

Besides this, there is a series of observations of the magnetic variation and also of the dip, at various times, from 1773 to August 1809 (Cavendish died Feb. 24, 1810).

These observations were made for the most part only in the summer months, but during that time were carried on with the greatest regularity, and results for each year calculated from

them.

We also find the record of "Trials of Nairne's needle in different parts of England in August, 1778.”

It was tried" in Garden, Aug. 8. In Garden of Observatory at Oxford, Aug. 14. At Birmingham, in Bowling-green, Aug. 15. At Towcester, in Garden, Aug. 17. At St Ives, in Garden, Aug. 18. At Ely, in Garden, Aug. 18. At London, Aug. 19 and 22.” From these trials he finds that "Lines of equal dip should seem to run about 44° to south of west, and dip should increase about 42' by going 1o to N.W."

There is a long and valuable series of experiments on the magnetic properties of forged iron, blistered steel, and cast iron. "Some bars were got from Elwell 31 inch long, 2.1 broad, and about 5 thick. On May 29, 1776, one of each was made magnetical, the marked end being the south pole. In trying the experiment the bars were placed perpendicularly against a wall 25 inches distant from the center of the needle, 91° to west of usual magnetic north, either the top or bottom of the bar being always on a level with the needle. They were kept constantly with the marked end upwards till after the observations of June 30, after which they were kept with the marked end downwards."

Cavendish determines in every case the "fixed magnetism and the "moveable magnetism" of the bar, and also its magnetism

when "struck 100 times on an anvil, falling 16 inches by its weight, and tried immediately after."

There are also 23 pages of experiments on the effect of heat on magnets, and a mathematical investigation of the bending of the dipping-needle by its own weight as affecting the determination of the dip, together with measurements of the elasticity of steel and of glass.

AN ATTEMPT

TO EXPLAIN SOME OF THE PRINCIPAL

PHÆNOMENA OF ELECTRICITY,

BY MEANS OF AN ELASTIC FLUID *.

BY THE

HONOURABLE HENRY CAVENDISH, F.R.S.

From the Philosophical Transactions of the Royal Society for 1771,

Vol. LXI. pp. 584-677.

[Read Dec. 19, 1771 and Jan. 9, 1772.]