ON THE METEOROLOGY OP TORONTO IN CANADA. 47 



Here also the general character shown by the comparison of the two sta- 

 tions is that of very close resemblance, while the minor differences also stand 

 out prominently. The greater variation to which the temperature is subject 

 at Toronto in March and April is very obvious in the column of range ; as is 

 also the small amount of the variation in the month of November at Prague. 

 The mean monthly range deduced from the twelve months is 4'3°"9 at Toronto, 

 and 39°*7 at Prague ; a considerable amount of difference, and which marks 

 the greater general vicissitude of the climate of Toronto : still it is deserving 

 of notice that Prague is occasionally liable to fully as great, and (during these 

 three years at least) even greater extremes of temperature than Toronto, as is 

 shown by the memorandum at the foot of the table ; it is indeed curious to 

 remark how very nearly the stations approach each other in the extreme 

 amount of their thermometrical range. July and August are the only months 

 in which during three years the observations at Toronto never show a tem- 

 perature of the air so low as the freezing point. At Prague there are five 

 months, viz. from May to September inclusive, in which during the three 

 years the temperature was never observed so low as 32° . 



If we seek in the old continent a station most nearly isothermal with To- 

 ronto, we must refer to a latitude considerably higher than Prague. The 

 station in M. Mahlmann's list (Dove, Repertorium, b. 4, and Humboldt, Asie 

 Centrale, tom. 3.), which most nearly resembles it in the mean temperature 

 of the different seasons, as well as in that of the whole year, is Wexio in 

 Sweden, in latitude 56° 53', and height above the sea 450 Parisian feet. 

 Toronto is in 43° 39', and height above the sea 330 English feet. The mean 

 temperatures are — 



Spring. Summer. Autimin. Winter. Annual. Coldest month.Wannest month. 



Aqueous Vapour. 



I proceed to consider the elastic force or tension of the aqueous vapour 

 contained in the atmosphere, and the degree of humidity produced by it, 

 together with the diurnal and annual variations of these phasnomena. 



The elastic force of the vapour is considered to be one of the constituents 

 of the pressure upon the surface of the mercury in the cistern of the barome- 

 ter, which, conjointly with the other and much larger constituent, viz. the 

 pressure of the gaseous atmosphere, produces what in common parlance is 

 called the pressure of the atmosphere, measured by the height of the mercurial 

 column in the barometer. Although we have no instrument by which we can 

 measure the gaseous pressure independently of that of the aqueous vapour, 

 we possess in Daniell's hygrometer, and in the wet and dry thermometers, the 

 means of ascertaining the aqueous pressure at any instant independently of 

 the gaseous pressure ; and therefore, by the combination of the barometer and 

 of the wet and dry thermometers (or of the hygrometer before mentioned), 

 we should be able to obtain separately the pressure due to each constituent, 

 and the annual and diurnal variations of both. It will be understood, there- 

 fore, that when the " tension of the vapour" is here mentioned, it expresses 

 also the pressure on the barometer produced by the elastic force of the vapour 

 present in the air. 



The scale in which the humidity of the iair is expressed is the simple 

 natural scale in which air at its maximum of humidity (i.e. when it is satu- 



