372 
had but + 0°097. This greater uncertainty arises from the 
strong contrast between the bright and dark surfaces whose 
boundary we take; and asimilar 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 flutter 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. 
‘* IT 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.” 
