M'Keevor's Voyage to Hudson's Bay. \Q 



azure hue ; it diminishes fast, as the atmosphere becomes 

 loaded with spreading clouds ; and it is almost extinguished 

 when low fogs settle on the surface. These effects are, no 

 doubt, more conspicuous in the finer regions of the globe. 

 Accordingly, they did not escape the observation of the an- 

 cients, but gave rise to opinions which were embodied in the 

 language of poetry. The term An?, was applied only to the 

 grossest part of the atmosphere, while the highest portion of 

 it, free from clouds and vapour, and bordering on the pure 

 fields of aether, received the kindred appellation of A*9 ft *. In 

 southern climates especially, a transpiercing cold is felt at 

 night under the clear and sparkling canopy of heaven. The 

 natives carefully avoid exposing themselves to this supposed 

 celestial influence, yet a thin. shed of palm-leaves may be suf- 

 ficient at once to screen them from the scorching rays of the 

 sun, and to shelter them against the chilling impressions 

 rained from the higher atmosphere. The captains of the 

 French gallies in the Mediterranean used formerly to coo! 

 their wines in summer by hanging their flasks all night from 

 the masts. At day-break they were taken down, and lapped 

 in several folds of flannel, to preserve them in the same 

 state. The frigorific impression of a serene and azure sky 

 must undoubtedly have concurred with the power of evapo- 

 ration in augmenting the energy of the process of nocturnal 

 cooling, practised anciently in Egypt, and now systematically 

 pursued in the higher grounds of India. As the chilliness 

 accumulated on the ground is greatest in clear nights, when 

 the moon shines brightest, it seemed very natural to impute 

 this effect partly to some influence emanating from that feeble 

 luminary. 



The instrument which Professor Lesslie employed in his 

 experiment on this highly interesting subject, he terms an 

 ./Ethrioscope (from the Greek word AiOfioo-, which, in reference 

 to the atmosphere, signifies at once clear, dry, and cold.) It 

 is, in fact, a combination of the ordinary pyroscope, and is 

 formed by adapting that instrument to the cavity of a polished 

 metallic cup, of rather an oblong spheroidal shape, the axis 

 being occupied by the sentient ball, while the section of a 

 horizontal plane, passing through the upper, forms the orifice. 

 The cup may be made of thin brass, or silver, either ham- 

 mered or cast, and then turned and polished on a lathe, the 

 diameter being from two to four inches, and the eccentricity 

 of the elliptical figure varied within certain limits, according 

 to circumstances ; the most convenient proportion, however, 

 ,is to have this eccentricity equal to half the transverse axis, 

 and consequently to place the focus at the third part of the 



JLJ '* 



