This result followed, however,—and it may be reckoned the second notable circumstance—that, upon his return, Mr. Bond constructed the model of an astronomical dome, the operative plan of which was the same as that of the great dome, built in 1844, and which has been in satisfactory use at Cambridge to the present time. The chief peculiarity of its mechanism is in the method of rotation by means of smoothly-turned spheres of iron. The dome rests on these at equi-distant points, and, being set in motion by suitable gearing, the iron balls sustaining its weight roll along a level circular track of iron, the circumference of which is equal to that of the dome. The method was unlike that previously in use. It appears to have been original with Mr. Bond, as is perhaps evinced by a remark in his report for 1848 referring to the matter: “If carefully examined, it will be found that this arrangement is as perfect in theory as it is appropriate and convenient in practice.” Experience has shown that spheres of hard bronze are more serviceable than those of iron, and bronze is now used.
The record indicates that an observatory did not cease to be a coveted object at any time during the 25 years prior to 1840. Two antecedent events, in themselves of importance, combined to bring the long cherished project to a happy issue,—the accession of Josiah Quincy to the presidency of the college and the action of Congress in authorizing what came to be popularly known as the “Wilkes Exploring Expedition.” The purpose of the expedition in part was to establish the latitudes and longitudes of uncharted places in distant parts of the world where American commerce was extending, and in part to investigate natural phenomena, including the facts of terrestrial magnetism. Having, after much delay, got an adequate appropriation, the naval department employed the best available talent of the country for the conduct of the enterprise.
Mr. Bond was engaged to make at his private observatory in Dorchester, Mass., investigations to fix a zero of longitude, whence final reference to Greenwich might be had, and to make a continuous record of magnetic observations at Dorchester for comparison with like records obtained at distant points by the expedition itself. As preliminary to the latter work, Mr. Bond tested in an isolated observatory in Dorchester the magnetic instruments with which the expedition was to be equipped.