346 Hydrostatics. 



2 a -f 2<7, 

 and for Z> its equal a d h', we shall have 



gAh' a$ = (2p /) X (2 a -f 2 d) ; 

 and, dividing by a and by g A, we have 



**/ = 2 !ZZ.' X 



#A 



and 



In applying this formula to the elevation of water between 

 two glass plates, the side a is very great compared with #, and 



therefore the quantity - being almost insensible, may be safely 

 neglected. Hence the formula becomes 



= 2 ^--x i 



8* " * 



By comparing this formula with the formula (in.) it is evident 

 that water will rise to the same height between glass plates, 

 as in a tube, provided the distance d between the two plates 

 is equal to R, or half the diameter of the tube ; in other words, 

 that, when the distance between the plates is equal to the diam- 

 eter of the tube, the elevation in the former case is half that in 

 the latter. This result was obtained by Newton, and has been 

 confirmed by the experiments of succeeding philosophers. 



2 p _ , pi 



As the constant quantity 2 -- is the same as that al- 



g A 



ready found for capillary tubes, we may take its value, name- 

 ly, 15,1311, and substitute it in the preceding equation ; we shall 

 then have 



15,1311 



^ 



and since 



