2. _State of the Atmosphere most proper for observing terrestrial and celestial objects._ The atmosphere which is thrown around the globe--while it is essentially requisite to the physical constitution of our world, and the comfort of its inhabitants--is found in many instances a serious obstruction to the accurate performance of telescopes. Sometimes it is obscured by mists and exhalations, sometimes it is thrown into violent undulations by the heat of the sun and the process of evaporation, and even, in certain cases, where there appears a pure unclouded azure, there is an agitation among its particles and the substances incorporated with them, which prevents the telescope from producing distinct vision either of terrestrial or celestial objects. For viewing distant terrestrial objects, especially with high powers, the best time is early in the morning, a little after sun-rise, and, from that period till about 9 o’clock A.M., in summer; and, in the evening about two or three hours before sun-set. From about 10 o’clock A.M. till 4 or 5 in the afternoon, in summer, if the sky be clear and the sun shining, there is generally a considerable undulation in the atmosphere, occasioned by the solar rays and the rapid evaporation, which prevents high powers from being used with distinctness on any telescope, however excellent. The objects at such times, when powers of 50, 70, or 100 are applied, appear to undulate like the waves of the sea, and, notwithstanding every effort to adjust the telescope, they appear confused and indistinct. Even with very moderate magnifying powers this imperfection is perceptible. In such circumstances, I have sometimes used a power of 200 times on distant land objects, with good effect, a little before sun-set, when, in the forenoon of the same day, I could not have applied a power of 50 with any degree of distinctness. On days when the air is clear, and the atmosphere covered with clouds, terrestrial objects may be viewed with considerably high powers. When there has been a long-continued drought, the atmosphere is then in a very unfit state for enjoying distinct vision with high magnifying powers, on account of the quantity of vapours with which the atmosphere is then surcharged, and the undulations they produce. But, after copious showers of rain, especially if accompanied with high winds, the air is purified, and distant objects appear with greater brilliancy and distinctness than at any other seasons. In using telescopes, the objects at which we look should, if possible, be nearly in a direction opposite to that of the sun. When they are viewed nearly in the direction of the sun, their shadows are turned towards us, and they consequently appear dim and obscure. By not attending to this circumstance, some persons, in trying telescopes, have pronounced a good instrument to be imperfect, which, had it been tried on objects properly illuminated, would have been found to be excellent. 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.