In our variable northerly climate the atmosphere is not so clear and serene for telescopic observation as in Italy, the South of France, and in many of the countries which lie within the tropics. The undulations of the air, owing to the causes alluded to above, constitute one of the principal reasons why a telescope magnifying above a hundred times can seldom be used with any good effect in viewing terrestrial objects--though I have sometimes used a power of nearly 200 with considerable distinctness, in the stillness of a summer or autumnal evening, when the rays of the declining sun strongly illuminated distant objects. The atmosphere is likewise frequently a great obstruction to the distinct perception of _celestial_ objects. It is scarcely possible for one who has not been accustomed to astronomical observations, to form a conception of the very great difference there is in the appearance of some of the heavenly bodies in different states of the atmosphere. There are certain conditions of the atmosphere essentially requisite for making accurate observations with powerful telescopes, and it is but seldom, especially in our climate, that all the favourable circumstances concur. The nights must be very clear and serene--the moon absent--no twilight--no haziness--no violent wind--no sudden change of temperature, as from thaw to frost--and no surcharge of the atmosphere with aqueous vapour. I have frequently found that, on the first and second nights after a thaw, when a strong frost had set in, and when the heavens appeared very brilliant, and the stars vivid and sparkling--the planets, when viewed with high powers, appeared remarkably undefined and indistinct; their margins appeared waving and jagged, and the belts of Jupiter, which at other times were remarkably distinct, were so obscured and ill-defined, that they could with difficulty be traced. This is probably owing to the quantity of aqueous vapour, and perhaps icy particles, then floating in the air, and to the undulations thereby produced. When a hard frost has continued a considerable time, this impediment to distinct observation is in a great measure removed. But I have never enjoyed more accurate and distinct views of the heavenly bodies than in fresh serene evenings, when there was no frost and no wind, and only a few fleecy clouds occasionally hovering around. On such evenings, and on such alone, the highest powers may be applied. I have used magnifying powers on such occasions with good effect, which could not have been applied, so as to ensure distinct vision, more frequently than two or three days in the course of a year.