ASSOCIATED WITH LACAILLE IN HIS LABOURS. 99 



refreshment of uninterrupted sleep. In our climates 

 during the inclement season, the sky is almost constantly 

 overspread by a thick curtain of clouds. Under pain of 

 postponing by some centuries the verification of this or 

 that theoretic point, we must watch the least clearing off, 

 and avail ourselves of it without delay. 



A favourable wind arises and dissipates the vapours 

 in the very direction where some important phenomenon 

 will manifest itself, and is to last only a few seconds. 

 The astronomer, exposed to all the transitions of weather, 

 (it is one of the conditions of accuracy,) the body pain- 

 fully bent, directs the telescope of a great graduated 

 circle in haste upon the star that he impatiently awaits. 

 His lines for measuring are a spider's threads. If in 

 looking he makes a mistake of half the thickness of one 



o 



of these threads, the observation is good for nothing ; 

 judge what his uneasiness must be ; at the critical mo- 

 ment, a puff of wind occasioning a vibration in the arti- 

 ficial light adapted to his telescope, the threads become 

 almost invisible ; the star itself, whose rays reach the eye 

 through atmospheric strata of various density, tempera- 

 ture, and refrangibility, will appear to oscillate so much 

 as to render the true position of it almost unassignable ; 

 at the very moment when extremely good definition of 

 the object becomes indispensable to insure correctness 

 of measures, all becomes confused, either because the 

 eye-piece gets steamed with vapour, or that the vicinity 

 of the very cold metal occasions an abundant secretion 

 of tears in the eye applied to the telescope ; the poor 

 observer is then exposed to the alternative of abandon- 

 ing to some other more fortunate person than himself, 

 the ascertaining a phenomenon that will not recur during 

 his lifetime, or introducing into the science results of 



