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Concerning the second part of this paper, namely, the causes which 

 often affect mirrors so as to prevent their showing objects distinctly, 

 though it be well known to astronomers that telescopes will act very 

 differently at different times, yet no particular inquiry had yet been 

 made respecting the cause of this imperfection. The experience our 

 author has acquired during his long series of observations, in which 

 he never lost sight of this circumstance, has enabled him to combine 

 a set of facts, from which he thinks himself authorized to deduce in- 

 ferences which will be found to throw a considerable light upon the 

 subject. 



These observations are here described at length, and arranged 

 under different heads, chiefly according to the state of the atmosphere 

 at the time they were made. Their results will in some measure 

 point out the nature of them. They seem to establish, as a general 

 principle, that in order to see distinctly with " telescopes, it is re- 

 quired that the temperature of the atmosphere and mirror should be 

 uniform, and that the air be impregnated with moisture." Hence it 

 appears that a frost after mild weather, or a thaw after frost, will 

 sensibly derange the performance of our mirrors, till either the frost 

 or the mild weather are sufficiently settled that the temperature of 

 the mirror, and indeed of the whole telescope, may accommodate 

 itself to that of the air. That when a frost, though very severe, be- 

 comes settled, the mirror will soon accommodate itself to the tem- 

 perature, and the telescope will be found to act well. That no tele- 

 scope brought into a cold atmosphere out of a warm room, can for a 

 time be expected to act properly ; and that no delicate observations, 

 with high magnifying powers, can well be made when looking through 

 a door, window, or slit in the roof of an observatory. It equally ap- 

 pears that windy weather in general, which must occasion a mixture 

 of airs of different temperatures, cannot be favourable to distinct 

 vision : and that the aurora boreales, when they induce, as they 

 often do, a considerable change in the temperature of the different 

 regions of air, are likewise detrimental as to distinctness. 



Sometimes the weather may be perfectly serene, and yet the tele- 

 scopes will act imperfectly. This may be owing to the dryness oc- 

 casioned by easterly winds, or by a change of temperature arising 

 from an agitation of the upper regions of the atmosphere, or perhaps 

 by both these causes combined together. 



Dry air, it seems, is by no means proper for vision ; and hence 

 dampness, haziness, and fogs, to a certain degree, will generally be 

 found favourable to distinctness : damp situations, therefore, and the 

 neighbourhood of lakes or rivers, need not be objected to in choosing 

 a spot for an observatory. As the warm exhalations of the roof of a 

 house in a cold night must disturb the uniformity of the temperature 

 of a certain contiguous portion of air, it is to be expected that the 

 appearance of stars seen over a house, and at no considerable distance 

 from it, will be affected by that emanation. 



Lastly, one of the most essential causes of the want of uniformity 

 in the performance of telescopes must, it seems, be ascribed to the 



