C 451 ] 
which had lately occurred to him. He was about to 
clean a little cup in which he kept oil, and he threw 
upon the water fome flies that had been drowned in 
theoil. Thefe flies prefently began to move, and turn- 
ed round on the water very rapidly, as if they were 
vigoroufly alive, though on examination he found 
they were not fo. 1 immediately concluded that the 
motion was occafloned by the power of the repulflon ' 
abovementioned, and that the oil ifluing gradually 
from the fpungy body of the fly continued the mo- 
tion. He found fome more flies drowned in oil, 
with which the experiment was repeated before us. 
To (hew that it was not any eflfedd of life recovered 
by the flies, 1 imitated it by little bits of oiled chips 
and paper cut in the form of a comma, of the lize of a 
common fly ; when the Arearn of repelling particles 
ifluing from the point, made the comma turn round 
the contrary way. This is not a chamber experiment ; 
for it cannot well be repeated in a bowl or difh of 
water on a table. A confiderable fur face of water is 
neceffary to give room for the expan fion of a fmall 
quantity of oil. In a difh of water, if the fmalleft 
drop of oil be let fall in the middle, the whole fur- 
face is prefently covered with a thin greafy film pro- 
ceeding from the drop j but as foon as that film has 
reached the fides of the difh, no more will ifluefrom 
the drop, but it remains in the form of oil, the fides 
of the difh putting a flop to its diffipation by pro- 
hibiting the farther expanfion of the film. 
Our friend Sir John pringle being foon after in 
Scotland, learnt there, that thofe employed in the 
herring fifhery, could at a diftance fee where the 
fhoals of herrings were, by the fmoothnefs of the 
M m m 2 water 
