go ESSAYS AND OBSERVATIONS 



bility arifes, and why it leaves no tradt of 

 moifture where it rolls. 



20. From the like reafoning, we may 

 conclude, That, w^hen a fmooth needle is 

 made to fwim, it does not any where touch 

 the water, but forms around it, by its repul- 

 sive power, a ditch or bed, whofe concavity 

 is much larger than the bulk of the needle. 

 [See Tab. iii. Fig. 3.] And hence it is eafy to 

 underftand how the needle fwims upon a fluid 

 lighter than itfelfj fince the quantity of 

 water, difplaced by it, may be equal to the 

 weight of the needle. Phammejia of this 

 kind, inftead of being reduced to hydrofta- 

 tical principles, are commonly attributed to 

 the mere tenacity of water, and even ufed 

 for meafuring its cohefive power. See Miiff- 

 cbe?ibroecki Elementa Phyfices, 



11. This inftance furnhhes us with a juft 

 and neceffary corredtion of the common hy- 

 , droftatical law, That " the whole fwimming 

 " body is equal in weight to a quantity of 

 " the fluid whcfe bulk is equal to that of 

 *' the part immerfed :" for, to comprehend 

 this, as well as all ordinary cafes, it fliould 

 be faid more generally, That *' the whole 

 *' weight of the fwimming body is equal to 



" the 



