32 ESSAYS anv. OBSERVATIONS. . 
19. AFTER it is thus proved by an op- 
tical argument that the drop is reaily not 
in contact with the plant which fupports 
it, we eafily conceive whence its wonder= 
ful volubility arifes, and why itleaves no 
tract of moifture where it rolls. 
20. From the like reafoning, we may 
conclude, That, when a {mooth needle is 
_madeto fwim, it does not any where touch 
the water, but forms around it, by its repul- 
five power, a ditch or bed, whofe concavity 
is much larger than the bulk of the needle. 
[See Tas. ili. Fig. 3-| And hence it is eafy 
to underftand how the needle fwims upon 
a fluid lighter than itfelf; fince the quan- 
tity of water, difplaced by it, may be equal 
to the weight of the needle. Phenomena 
of this kind, inftead of being reduced to 
hydroftatical principles, are commonly at- 
tributed to the mere tenacity of water, 
and even ufed for meafuriug its cohefive 
power. See Muf[chenbroeck, Elementa Phyf- 
£e5. 
21. Tus inftance furnifhes us with a 
juft and neceffary correCtion of the com- 
mon hydroftatical law, That “the whole 
“ fwimming 
