done, he believed himself to be the originator; and mastered the English translation by Motte of Newton's "Principia."
The acquaintance which he formed in 1751 with Thomas Barton, who afterward married his sister, had an important influence in shaping his career. Rittenhouse, according to William Barton, "possessed a sublime native genius; which, however, was yet but very imperfectly cultivated for want of indispensable means of extending the bounds of natural knowledge." Barton had enjoyed these means, and had acquired the reputation of being a man of learning. He found Rittenhouse's society profitable, and Rittenhouse found his equally so. Barton aided Rittenhouse greatly by helping him to the books he needed. Partly through his instrumentality a circulating library was established at Norriton; and he bought books for Rittenhouse when he went to Europe.
Mr. Rittenhouse was called upon in 1763 to determine the initial of the boundary-line between Pennsylvania and Maryland, his particular duty being defined to be to ascertain and fix the "circle to be drawn at twelve miles' distance from New Castle, northward and westward, with the beginning of the fortieth degree of north latitude," etc. The work was an arduous one, and involved going through a number of tedious and intricate calculations. It was performed in a satisfactory manner, for which acknowledgment was made in the shape of extra compensation, and with instruments to a large extent of Rittenhouse's own making; and his observations were accepted without change by the official astronomers, Mason and Dixon, when they took charge of the work. He was afterward appointed to a similar work in 1769, by the commission to settle the boundary between New York and Pennsylvania. Among his scientific studies at this period were the investigation of variations in the oscillations of the pendulum under changes of temperature, with the device of a plan for compensation, and the construction of what he called a metalline thermometer. This instrument was so made—on the principle of the expansion and contraction of metals under variations of temperature—that the degrees of heat and cold were indicated by the movements of an index moving along a graduated semicircle. It was adapted, in form and size, to be carried in the pocket. He discussed the compressibility of water in the light of an experiment that had been reported to the Royal Society, and observed, in a letter to Mr. Barton, that, although the experiment did not please him, he did not doubt the fact; for, "if the particles of water were in actual contact, it would be difficult to conceive how any body could much exceed it in specific gravity; yet we find that gold does, more than eighteen times." We find him also at this time (1767) indulging in some amusing speculations on the possibility of a man's moving the world.
