162 Professor Airy on the Longitude 



4 seconds it will beat exactly with the chronometer, and in 4 

 seconds more it will be j, of a second before the chronometer. 

 During these 8 seconds the beats will strike the ear at so short 

 an interval that no distinction of sounds is perceptible. The 

 business of the observer, therefore, in comparing a chronometer 

 is to note down the time shewn by both at one of these seconds 

 in which no interval of sound can be perceived. He must then 

 wait till the sidereal clock has gained so much on the chronometer 

 that another coincidence of beats can be observed. 



Our chronometers beat 5 times in 2 solar seconds, and con- 

 sequently the coincidence of beats took place at every alternate 

 second for a short time. The next set of coincidences took place 

 when the clock had gained \ of a second on the chronometer, 

 or at the end of about 73 seconds. 



The accuracy of this mode of comparison can scarcely be 

 imagined without trial. I think there is no doubt that a practised 

 ear can determine the instant of coincidence with an error cer- 

 tainly not exceeding 8 or 9 seconds. This implies an error in 

 the comparison not exceeding ^ of a second. When three or 

 more comparisons are made, and the mean taken, it is probable 

 that the error seldom amounts to jj^ of a second. 



I think it unnecessary to set down at full length every one 

 of the comparisons of the chronometers and clocks, and shall 

 only give the mean of those taken in immediate succession. It 

 is sufficient to state that the number of comparisons has never 

 been smaller than three, and that five have sometimes been used : 

 that these have been carefully examined by differences to dis- 

 cover the errors which almost inevitably occurred in noting them 

 down: that in one or two instances errors have been discovered 

 and corrected : that the means have been compared with the 

 observations which were nearest in point of time, as a check 

 on the accuracy of the means ; and that sometimes the same 



