30 



NATURE 



[May 14, 1874 



The same remarks apply to the heliometric method. 

 But willi ^t1tlon5 chosen where the sun is not too low, we 



^ 



may expect accurate results. The value of a heliometer 

 over other instruments designed for measuring small 



angles consists in this, that by it we can measure angles 

 as large as the sun's diameter. It is expected by obser- 

 vers with this method that an observation will be made 

 each time with an accuracy comparable with that of an 

 observation of the time of contact. In this case the 

 heliometric method will give valuable results. For the 



same reasons observations made by means of a double- 

 image micrometer of the distance between the limbs of 

 the sun and Venus near the time of contact will be as 

 accurate as an observation of the contact itself. 



The last difficulty which we shall mention in connec- 

 tion with this kind of observation is due to atmospheric 



condilions as affecting the apparent time of contact. With 

 regard to the British expedition, great care has been 

 taken to choose stations where the weather can be de- 

 pended upon. But in cases where the method of dura- 

 tion is applied, the observations will be useless if there be 

 not a very clear atmosphere both at ingress and at egress. 



Da I'Isle's method, on the other hand, requires a perfect 

 observation only at the time of one of these phases. 

 Hence the nations which have adopted this method are 

 less likely to be disappointed than others. 



George Forbes 

 ( To he continued.) 



