502 
ME. W. CEOOKES ON ATTEACTION AND 
3. Many physicists have worked on the subject of repulsion by heat. I give here a 
brief resume of the state of knowledge on this subject up to the time of my com- 
mencing these experiments, premising, however, that much of this historical informa- 
tion was unknown to me until some of the experiments here recorded were finished 
and I commenced putting my notes together. The earliest mention I can find is by 
the Rev. A. Bennet, F.R.S., who in the year 1792 published a paper* on “ A new 
suspension of the magnetic needle intended for the discovery of minute quantities of 
magnetic attraction : also an air-vane of great sensibility ; with new experiments on 
the magnetism of iron filings and brass.” Mr. Bennet used a spider’s thread as a 
means of suspension. This he found by experiment to be absolutely free from torsion. 
I quote the following experiments from his paper: — 
“ Experiment IV. A bristle was suspended horizontally by a spider’s thread, some- 
what stronger than the last, and after turning the wheel till it produced 4800 revolu- 
tions, it shortened the thread from 3 inches to 1 inch ; yet either end of the bristle 
would move towards any warm substance which was presented to it either with or 
against the direction of the twist f. 
“ Experiment V. Several other light substances were suspended by fine spiders’ threads 
and placed in a cylindrical glass about 2 inches in diameter, as the thinnest part of the 
wing of a dragon-fly, thistledown, and the down of dandelion ; of these the last appeared 
most sensible to the influence of heat ; for when this down was fastened to one end of a 
fine gold wire, suspended horizontally on to one end of two bits of straw joined together 
in the form of the letter T inverted, it would turn towards any person who approached 
it at the distance of 3 feet, and would move so rapidly towards wires heated by my 
hand, as very much to resemble magnetic attraction. 
“ Experiment VI. A bottle filled with cold water was brought near the glass cylinder 
standing in a warm room, and soon after the down of dandelion appeared to be repelled 
by the bottle by turning away from it. The bottle was removed to the other side, and 
the dandelion again moved towards the opposite side. 
“ Experiment VII. A piece of paper was tied over the mouth of a glass jar, about 
4 inches in diameter. Two holes were made in the paper opposite to each other, and 
near the edge of the glass. The jar was placed upon a table, and suffered to stand a 
considerable time to cool in a room without fire. I then sat near it on the side where 
one of the holes in the paper was in the nearer and the other in the farther end of the 
diameter. I next filled another glass with smoke, and placed it with its mouth over 
the two holes in the paper. The smoke was now seen to descend through the farthest 
hole, and mixing with the air in the lower jar, plainly showed that the air moved slowly 
towards the side of the glass warmed by the heat of my body. 
“ Experiment X. To the end of a fine gold wire 3 inches long, and suspended by a 
spider’s thread in a cylindrical glass, was fastened a small circular bit of writing-paper ; 
* Philosophical Transactions, 1792, p. 81. 
t Por a rediscovery of this fact, seventy-nine years after, see par. 14. 
