Dr* Wall on AttraSfion and Repulfwn. 46 1 



(fnagnetical, or eledlrical attraftion or repulfion. 

 Thefe, efpecially with refpedt to magnetifm and 

 eledricity, are perfedlly cftablifhed. Whether 

 there is any mechanical repulfion, independent 

 of thefe, or the eleftive attraction, of which I 

 have faid fo much, may perhaps be difputed. 



"By this referve, I think, I obviate your objec- 

 tion to my theory, drawn from Mr. MehiU's,, or 

 Muffchetibroek's experiment of the needle. Pro- 

 bably, this phaenomenon is conneded with othef 

 principles, than thofe which chemiftry will fup- 

 ply. The fufpenfion of the needle depends upon 

 its exad level pofition on the furface of the 

 water, and upon the great extent of the furface, 

 when compared with the bulk of the needle and 

 the quantity of water, which it muft. difplace 

 when it finks. Only the lower fide of the 

 needle touches the water, and no part of the upper 

 fide is wetted at all ; for if any part of the upper 

 fide be fo wetted that either end is deprefiTed, 

 the gravity of the different parts of the needle 

 not ading feparately, but in combination, the 

 whole overcomes the refiftance of the water, 

 and the needle finks. In the fame manner, 

 many fubftances, fpecifically heavier than water, 

 may be made to float upon water by confiderably 

 extending the furface. Inftances of this fa6t 

 are fo numerous, that they need not be men- 

 tioned; and that this is really the cafe in the 

 experiment under confideration, is obvious from 



this 



