Various improvements of method had been made, and that most approved was followed on this first occasion at Cambridge. The coast survey officer in charge had for his assistants Prof. Bond at Cambridge and Prof. Loomis in New York. An official letter of the electrician of the Coast Survey Department says: “During these experiments Prof. Bond conceived the idea of using an automatic circuit interrupter.” Some question of priority as to this suggestion arose in later years. It was doubtless a spontaneous and original thought with Prof. Bond, though the suggestion appears to have been made earlier elsewhere, but it had not been acted upon “from apprehension of injury to the performance of an astronomical clock which must be used for the purpose.” Experience proved eventually the apprehension to have been groundless; but Prof. Bond’s suggestion avoided any liability of the kind by proposing that an astronomical clock be made for the purpose. In August, 1848, he received authority to have such a clock made at the expense of the coast survey. Reverting to the matter in a subsequent annual report, Prof. Bond says: “I caused such a clock to be made, and it is found to answer perfectly the intended purpose. But another and far more serious difficulty presented itself in the accurate registry of the beats of the clock after being transmitted by the galvanic circuit; and it was at this point that further progress in the application of this method to astronomical observing was arrested.” [Illustration: DAVID SEARS. The First Donor to the Observatory.] Experimenters were busy at Philadelphia, Cincinnati, and elsewhere, during the two years’ interval in attempts to solve this concomitant problem, and with very considerable success. That none of these devices quite filled the requirements is manifest by the fact that they did not go into general use. But the perfected apparatus submitted to the officer of the coast survey by Prof. Bond, April 12, 1850, did go into such use. This instrument Prof. Bond stated to be the joint invention of himself and his two sons, George P. and Richard F. Bond. It was named at first from one of its peculiar parts, the “spring-governor,” but the more comprehensive title of “chronograph” was later applied to it. While as a piece of mechanism, it was distinct from the “circuit interrupter,” the two were used conjointly, and thus acting in combination their operation in recording became known soon afterwards in England as “the American method.” By this method the errors suggested by the term “personal equation” are greatly diminished, and a definiteness of record is attained, which permits the recording sheet to be read by the eye to tenths and by scale and lens to hundredths of a second. The successive sheets are the primary official record, and being bound into volumes, become a part of the permanent archives.