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given to the second. The American astronomers have com- 

 pared the results of this method with the Greenwich transit 

 observations, and have found, in the words of Mr. Airy, that 

 ' the measure of its irregularities is only about one- fourth of 

 that of the other.' He, however, very truly remarks that 

 ' a portion of these differences may be owing to the difference 

 in the state of the atmosphere, that of England being perhaps 

 comparatively unfavourable to accurate observations.' 



" In making a comparison of this kind it must be remem- 

 bered that the two systems not merely employ different senses 

 to co-operate with sight, but that the sight itself is used in 

 two different ways. In the ' Electric Register' the eye catches 

 the moment when the star crosses a wire, and the touch is to 

 synchronize with it in giving a signal. In the common modes 

 the ear counts the clock's beats ; the eye notes the distances 

 of the star from the wire at the beats which precede and fol- 

 low its passage, and thus the mind estimates the fraction of 

 the second from the relative position of three points ; all ex- 

 isting in memory alone when that estimation is made. Now 

 this is rather a complicated operation ; and it is evident that 

 the optical part of it must be much more liable to uncertainty 

 than the mere noting the occurrence of the transit. Of this 

 latter, as well as of the relative promptitude of hearing and 

 touch, it is desirable to have careful determinations ; but if 

 any such have been made during the five years that have 

 elapsed since this new principle of observation was discovered, 

 they have not reached me. In hopes, therefore, of drawing 

 attention to the subject, I offer to the Academy the results of 

 a similar examination, applied to the Armagh Transit Obser- 

 vations, for the purpose of ascertaining how far they could be 

 improved, and what probable weights should be given to certain 

 elements of reduction. 



" The error which may be caused by uncertainty in catch- 

 ing the precise part of the beat which shall be taken for the 

 origin of the second, is the same for every star (at least if we 



