48 



REPORT — 1844. 



rated with vapour) is reckoned as = 100 ; and air absolutely deprived of 

 moisture as = : the intermediate degrees are given by the fraction 

 100 X actual tension of vapour, 



tension required for the saturation of the air at its existing temperature. 



Thus if the air at any temperature whatsoever contains vapour of half the 

 tension which it would contain if saturated, the degree is 50; if three- 

 fourths, then 75 ; and so forth. 



Air of a higher temperature is capable of containing a greater quantity of 

 vapour than air of less temperature ; but it is the proportion of what it does 

 contain to what it would contain if saturated, which constitutes the measure 

 of its dryness or humidity. 



The capacity of the aii- to contain moisture being determined by its tera- 

 pei'ature, it was to be expected that an intimate connexion and dependence 

 would be found to exist between the annual and diurnal variations of the 

 vapour and of the temperature. I shall proceed to show how distinctly and 

 fully this connexion is exhibited by the observations at Toronto. We will 

 commence with the humidity. 



Diurnal Variation. — The degree of humidity at tlie several observation 

 hours exhibits, as in the case of the temperature, a simple progression of one 

 ascending and one descending branch, having its turning points the same 

 as those of the temperature, namely, a maximum at or near the coldest, 

 and a minimum at or near the hottest hours of the day ; the progression is 

 inverse, but is in harmony with that of the temperature. 



Mean degree of Humidity at Toronto at the several Observation Hours. 



The accord of the two years' observations is remarkably satisfactory ; they 

 unite in showing that in the average state of the atmosphere at Toronto, the 

 air is charged Avith between three-fourtlis and four-fifths (or more exactly 

 with 78 parts in 100) of the vapour required for its saturation. 



When we proceed to the mean tension of the vapour at the several obser- 

 vation hours, we perceive an accord with the march of the temperature fully 

 as striking ; one ascending, one descending branch ; — the turning points in 

 obvious dependence, — and the march hannonious ; in this case the progression 

 is direct, in relation to that of the temperature, — as it was inverse in the case 

 of the humidity. 



Mean Tension of the Vapour at the several Observation Hours. 



The direct evidence of connexion and dependence exhibited in the diurnal 

 march of the vapour and temperature at Toronto is the more deserving of 

 our notice, because in many climates, this connexion, though it always exists, 



