A VISIT TO AN OBSERVATORY. 101 



available, but a careful forethought must be exercised 

 in choosing the appropriate times for each object. But 

 even when the best moment for observation has arrived, 

 and we are quite ready to secure the examination of the 

 celestial body under the most favourable auspices, there is 

 still the very important consideration of the weather. For 

 astronomical work a clear sky is indispensable. When we 

 find that a cloud or two can obscure from us the direct 

 image of the sun, we need hardly expect that stars or 

 planets or objects still fainter and more delicate will be 

 perceived through a dense curtain of watery vapour. 

 Fogs and mists, no less than more extensive clouds, must 

 be absent, and even nights that seem tolerably clear to the 

 ordinary spectator may, from atmospheric causes alone, be 

 very ill-adapted for any careful astronomical work. 



From all these reasons it will be seen that the hours in 

 which really excellent work can be done in an observatory 

 are but few in comparison with the whole number of hours 

 in the year. When such hours do arrive astronomers 

 greatly prize them, and it may be readily believed that 

 during the time for which they have waited so long 

 they are often not too well pleased to be disturbed by 

 visitors who come on a star-gazing expedition. 



Of all the impediments to astronomical work those pro- 

 duced by the atmosphere are the most vexatious, because 

 they do not admit of being predicted. The astronomer 

 may sometimes have even gone to the other side of the 

 earth to observe some rare phenomenon like a total eclipse 

 of the sun, or the still more occasional transit of Venus 

 across the sun. The time when such an event will occur 

 admits of actual prediction ; all may be in readiness, when 



