CKI.KSTIAL DAY OBiKRVATlONS. 
127 
ing, especially if llieir aliiiudes are low : and in the northern Observations 
part of the heavens than in the southern. The ditl'iculty f**?' 
perceiving them, in the first case, is obviously owing to the the tljy-uiue. 
thick vapours near the horizon : in the second case, to the un- 
dulation of the air, which is generally greater in the morning 
than in the afternoon. This may very evidently be ptr- 
ceived by looking at a dutant land object, at those time.s. In 
a hot day, through a telescope that magnifies 30 or 40 times: 
in the third case it is owing to the northern part of the sky 
being of a deeper azure on account of its being less enlightened 
jhan the southern, with the splendour of the so ar rays- 
Had I not already protracted this communication too far, I 
•ntended to have offered a few remarks on the utility of the 
equatorial telescope, and the practical uses to which observa- 
tions on stars, in the day-time, m ly be applied. I shall only at 
■present observe, that in accurately adjusting circular and transit 
instruments, it is useful, auJ even necessary, for determinirg 
the exact position of the meridian, to take observations of seve- 
ral stars, which differ greatly in zenith distance, and which 
transit the meridian nearly at the same time. But as tlie stars 
best situated for this purpose, cannot, at every se.ison, be seen 
in the evenings, we must, in certain cases, >Aait for several 
months till suc h observations can be made, unless we make them 
in the day-time, which can very easily be done, if the instru- 
ment has a telescope adapted to it, furn shed wiih such powers 
as those above stated. I have also (requeiiily made use of ob- 
servations on the stars, in the day-lime, for adjusting a clock or 
watch to mean time, when the sun was in a silu.uion bevond 
the range of the instrument, or obscured by clouds ; and 
when I did n<rt chuse to wait till the evening. This may, at 
first view, appear to some as paradoxical, since the finding of 
a siai in day-light depends on our knowing its right ascension 
from the sun, and iliis las circumstance depends on cur know- 
ing the true time. But if a watch or a cluck is known not to 
have varied above seven or eight minutes from the time, a star 
of the first magnitude may easily be found by moving the teles- 
cope a little backwards or forwards, till the star api>ear j and 
when 
