It 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 sufficiently concise, and sufficiently elementary, to admit of its being with propriety introduced into a course of academical instruction, has been hitherto, so far as I know, a desideratum. The following may perhaps be found to possess that advantage.