367 
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 asignal. 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 ix 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 
