372 



had but ± 9, 097. This greater uncertainty arises from the 

 strong contrast between the bright and dark surfaces whose 

 boundary we take ; and a similar one may be expected in at- 

 tempting to note the precise instant of its passage. 



" As to tremor, it will act here precisely as in transit ob- 

 servations, and it may, moreover, perhaps nullify one of the 

 proposed advantages, that of making many observations in a 

 few seconds. The undulations of the air are twofold, those of 

 short period, which cause the nutter that produces z, and 

 those of much longer duration, which, without blurring the 

 star, displace it, and sometimes cause the pole-star, after cross- 

 ing a wire, to go back, and after ten or even fifteen seconds 

 make a second transit. In the present mode such waves 

 affect only a single wire, and may be neutralized by others ; 

 but in the other they would vitiate the whole set. 



" I mention these doubts in the hope that some of our own 

 astronomers may take up the subject, and examine it fully. 

 If there be practical objections to it they may be remedied; 

 but if it really possess the advantages which it seems to have, 

 it should be decidedly adopted. At all events it is a step in 

 the right direction, for we have now carried the existing pro- 

 cesses and instruments of astronomical research nearly as far 

 as they can go, and new powers must be invoked, if we wish 

 to make further progress." 



