a ET el 
XII. Elementary Demonstration of the Composition of Pres- 
sures. By Tuomas Jackson, LL.D. F.R.S. Eni. 
and Professor of Natural Philosophy in the Universi-- 
ty of St. Andrew’s. 
(Read June 3, 1816.) 
T is well-known as a fundamental principle in statics, that’ 
“« Two pressures, represented in direction and quantity by 
“ two adjoining sides of a parallelogram, are equivalent to one 
“ represented in direction and quantity by the diagonal 
“ which passes through the point at which these sides meet.” 
A demonstration of this proposition, that shall be at once suf- 
ficiently concise, and sufficiently elementary, to admit of its be- 
ing with propriety introduced into a course of academical in-. 
struction, has been hitherto, so far as I know, a desideratum.. 
The following may perhaps be — to possess that advan-. 
tage.. : 
Lema. 
“ If the equivalent of two pressures, represented by the ad- 
“ joining sides of a rectangle, given in species, be always re- 
“« presented in direction by the eee gee diagonal, it's shall 
“ be represented by the same in quantity.” 
Let 
