Memoirs of the Lit. and Phil. Society of' Manchester. 157 



ascertaining whether or not there existed a distinct vapour atmosphere, 

 mechanically hlended with the common one, but acting by its own ten- 

 sion or elasticity, and being subject to condensation by cold exactly in the 

 same manner as an insulated atmosphere of steam would be. Mr. Dal- 

 ton's observations, however, though original in their nature and design, 

 do not in his own opinion establish the existence of such an atmosphere. 

 The following are the results which their author has deduced from them : 



1. That the quantity and density of vapour is constantly (or with 

 very rare exceptions) less the higher we ascend. 



2. That wherever a dense cloud or fog exists, then the temperature 

 of the air and the dew point are the same. 



3. That when a mountain is wholly, or in great part, enveloped in 

 fog, there is little variation in ascending, either in the temperature of 

 the air, or in the dew-point. 



4. That, upon an average, the temperature of the air sinks after the 

 rate of 1° for every 80 yards of perpendicular ascent, about the middle, 

 or warmest part of the day; and that of the dew-point 1° for every 130 

 yards perpendicular ascent. 



5. That the phenomena of aqueous meteors, such as rain, fog, dew, 

 &c. depend upon the known relations of heat and water, and are ex- 

 hibited to us in miniature every day in our domestic economy. Elec- 

 tricity appears to be a consequent, rather than an agent, in the forma- 

 tion and decomposition of clouds ; or, if a necessary agent, it is equally 

 so in the boiling of water, or in the drying of piece-goods in a stove. 



Mr. Dalton concludes his paper with the following useful Table, 

 which exhibits in numbers the drying power of the air, according as its 

 temperature is elevated above that of the dew-point. 



The next paper in this volume is a very elaborate and interesting one, 

 entitled, Tables of the various Species of Periodical Birds observed in the 

 neighbourhood of Manchester, with a few Remarks, tending to establish 

 the opinion that the Periodical Birds Emigrate ; by Mr. John Black- 

 wall. The following tables will exhibit Mr. Blackwall's arrangement 

 of periodical birds, and the principal facts which he has observed. 



Table I. — Periodical Summer Birds. 



Appear. 

 Sand Martin Hirundo riparia April 6 



Wryneck i r unx torquilla do. 



Water Wren Motacilla trochilus do. 12 



Disappear. 

 Sept. 16 



do. 12 



