eS ee ee Pe 
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 
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 
