C 363 ] 
The greateft weight I have been able to raife by 
the force of electrical cohefion, as appears above, has 
been feventeen ounces. Now the white flocking, 
which weighed but 17 dwt. and an half, bore all this 
weight: in this cafe therefore it railed, by the ftrength 
of its cohefion with the black, 340 pennyweight ; 
that is, nearly twenty times its own weight *. And 
if we confider that the force, applied to feparatc them, 
aCted in a direction parallel to the furfaces, by which 
they cohered; and that when the furfaces are fmooth, 
a force aCting in fuch direction, has much greater in- 
fluence in feparating bodies, by making them Aide 
gently over one another, than if thofe bodies were 
rigid, and the force employed to feparate them aCted 
in a direction perpendicular to the cohering furfaces ; 
* As the experiments mentioned above were made in the (pace 
of fix or feven weeks from about the latter end of March, when 
the temperature of the air begins to be lefs favourable for eledtrici- 
ty, I did not doubt, but upon the approach, and during the progrefs 
of the enfuing winter, I fhould meet with inftances of a ftronger 
degree of cohefion, than I had before been able to afcertain. Ac- 
cordingly, fince this paper was read in the Royal Society, and par- 
ticularly in the months of November, December, and January laft, 
at times, when the weather was clear and frofty, I found that the 
fame ftockings lifted confiderably more, than I had been able to make 
them do in the preceding months of March, April, and May. I 
likewife found, that when the ftockings were perfe&ly new, or the 
black dipt afrefh, and the white newly cleaned and fulphured, as 
alfo, that when they were of a more fubftantial make, fuch as thofe 
that are wove of fpun filk, weighing commonly about the double 
of thofe that go by the name of half gauze, their power of cohe- 
fion, when favoured by the temperature of the air, increafes to a 
very confiderable degree. Under thofe circumftances, at particular 
times, I have been able to make the black flocking, or the white, 
when the rough fides of each were put together, raife (the half 
gauze) from 20 to 40, and (of fpun iilk) from 40 to 90 times its 
own weight. Vide the fubfequent letter from Dr. John Mitchell 
to the Rev. Dr. Birch. 
A a a 2 
when 
