compensation by attaching to the pendulum a bent tube of glass, partially filled with alcohol and mercury. In 1767 he wrote a paper for the Pennsylvania Gazette upon the famous problem of Archimedes, and made some experiments upon the compressibility of water, reaching the conclusion, notwithstanding the tests of the Florentine Academy, that it was compressible. The same year he made a thermometer based upon the principle of the expansion and contraction of metals. An index moved upon a flat surface over a semicircle, which was graduated according to the Fahrenheit degrees of heat. During the present century Breguet has obtained much reputation by inventing anew this forgotten instrument.
A greater mechanical design was, however, now in contemplation than any he had before undertaken. He conceived the idea of endeavoring to represent by machinery the planetary system. Similar attempts had previously been made, but all had represented the planetary movements by circles, being mere approximations, and none were able to indicate the astronomical phenomena at any particular time. The production of Rowley, a defective machine, giving the movement of only two heavenly bodies, was bought by George I. for a thousand guineas. Rittenhouse determined to construct an instrument not simply to gratify the curious, but which would be of practical value to the student and professor of astronomy. After three years of faithful labor, in the course of which, refusing to be guided by the astronomical tables already prepared, he made for himself the calculations of all the movements required in this delicate and elaborate piece of mechanism, he completed, in 1770, his celebrated orrery. Around a brass sun revolved ivory or brass planets in elliptical orbits properly inclined toward each other, and with velocities varying as they approached their aphelia
