SKETCH OF SEARS COOK WALKER. 119 



Peirce was at the same time calculating the planet's perturbations. 

 The approximate results of each furthered the computations of 

 the other, so that within eighteen months from the discovery of 

 the planet these two Americans had attained a remarkably ac- 

 curate statement of its theory. 



In conjunction with Prof. A. D. Bache, Superintendent of the 

 Coast Survey, Walker developed the method of determining dif- 

 ferences of longitude by telegraph. "What was the separate share 

 of each of these two men in this work will probably never be known, 

 for each ascribed the chief merit to the other. One feature intro- 

 duced by Walker was the application of the method of coinci- 

 dence of beats to the comparison of timekeepers one indicating 

 mean, the other sidereal time at the two ends of a telegraphic line. 

 These beats were signalized from one station to the other by taps 

 of an observer upon the telegraph key. Such signals are, of 

 course, subject to the errors that always attend the action of 

 human nerves and muscles, so the next problem was to make the 

 clock give its own signals. Two methods had been proposed, but 

 there were fears groundless they have since been proved that 

 either of these would injuriously affect the running of the clock. 

 Mr. Walker sought diligently for some apparatus that would not 

 arouse any such fears. He propounded the problem to several 

 astronomers, and two or three contrivances were devised for the 

 purpose. 



This mode of observation and the apparatus invented to meet 

 its requirements proved valuable not alone for determinations of 

 longitude, but also for all other astronomical observations requir- 

 ing minute precision in the determination of time. The mental 

 effort required of the observer being reduced to a minimum, 

 many more transits could be observed at a single meridian pas- 

 sage. Walker immediately modified the transit instrument to 

 suit the new requirements, and, instead of five, seven, or at most 

 nine threads, he provided it with several tallies of five threads 

 each. There remained but one requisite to complete the Ameri- 

 can method of observation. This was some mechanical contriv- 

 ance for securing a uniform rotary motion of the record sheet. 

 It had not been attained when Walker died, although some prog- 

 ress toward the solution of the problem had been made. 



It is proper for the biographer to point out the share which 

 Walker personally had in this series of inventions, although he 

 was far from making any such claims for himself. With a fine 

 comradeship he was jealous only for the credit of the organization 

 of which he was a member the United States Coast Survey. 

 Speaking to the American Association for the Advancement of 

 Science, Walker said: "With the single exception of the ex- 

 periment between Baltimore and Washington, in 1844, I know of 



