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secute them more extensively, and during the past summer compa- 

 risons have been made between New York and Cambridge observa- 

 tory near Boston. The plan of operation this season was more 

 matured than during the former. The comparisons were all made 

 between a solar chronometer at Cambridge and a sidereal clock at 

 New York. At ten o'clock in the evening, the two observatories 

 having been put in telegraphic communication, when the seconds 

 hand of the solar chronometer came round to 60^ a signal was given 

 at Cambridge, by pressing the key of the telegraph-register ; at the 

 same instant a click was heard at New York, and the time was re- 

 corded according to the sidereal clock. At the end of 10^ a second 

 signal was given, which was also recorded at New York ; at the end 

 of another 10' a third signal was given, and so on for sixty seconds. 

 The Cambridge astronomer then commenced beating seconds by 

 striking the key of the telegraph-register in coincidence with the 

 beats of his chronometer. The New York astronomer compared the 

 signals received with the beats of his clock, and waited for a coin- 

 cidence. When the beats were sensibly synchronous the time was 

 recorded, and the astronomer waited six minutes for another coin- 

 cidence of beats. The Cambridge astronomer continued beating 

 seconds for fifteen minutes^ during which time the New York ob- 

 server was sure of two coincidences, and might obtain three. When 

 these were concluded, the New York astronomer in the same man- 

 ner gave signals for one minute at intervals of 10% and then beat 

 seconds for fifteen minutes, during which time the Cambridge astro- 

 nomer obtained four or five coincidences upon his chronometer. 

 This mode of comparison was practised every night, and it is con- 

 sidered that, the uncertainty in the comparison of the time-pieces 

 cannot exceed two or three hundredths of a second on any night; 

 and in a series of comparisons the error may be regarded as entirely 

 eliminated. 



Another mode of comparison which was practised is that of tele- 

 graphing star transits. A list of stars which culminate near our 

 zenith at intervals of five or six minutes was prepared, and the ob- 

 servers, both at New York and Cambridge, were furnished with a 

 copy. They then proceeded as follows : Cambridge selected two stars 

 from the list, which we will call A and B, and struck the key of his 

 register at the instant when the star A passed each of the seven wires 

 of his transit. These signals were heard at New York, and the times 

 recorded. Cambridge then observed the transit of star B in the 

 ordinary manner without telegraphing. New York then observed 

 the transit of star A on his meridian in the usual manner ; and struck 

 his key at the instant the star B passed each of the seven wires of 

 his transit, which signals were heard and recorded at Cambridge. 

 The difference of longitude between New York and Cambridge is 

 nearly twelve minutes, affording ample time for all these observations. 

 Thus New York obtained upon his own clock the times of transit of 

 star A over the meridians of Cambridge and New York ; and Cam- 

 bridge obtained upon his chronometer the times of transit of star B 

 over the same meridians. The difference of these times gives the dif- 



