104 Lieut.- Col. Sabine's Report on the Meteorology of Toronto. 



Mean Monthly Pressure. 



In turning our attention to the column which exhibits the excess or defect 

 of the mean monthly pressure on the mean of all the months, we at once 

 perceive another illustration of the principle which has been just stated. We 

 find the pressure of the gaseous atmosphere diminished in the summer months 

 and augmented in the winter months. The general dependence on the 

 march of the temperature is manifest ; and it must remain for the additional 

 evidence which will be produced by the observations of subsequent years, to 

 determine, whether the minor deviations from a perfectly harmonious march 

 are mere accidental differences, which a wider observation basis will cause to 

 disappear, or whether they may not point to some other periodical influence 

 (possibly of the temperature also, but of a less direct nature) which is as 

 yet unrecognized*. 



I will now ask the Section to turn its attention for a moment to the column 

 which presents the mean height of the barometer in each month of the 

 year. It is curious to observe how completely the annual march of the gaseous 

 atmosphere is masked in the barometer by its combination with the vapour 

 pressure, both being measured in one by the mercurial column ; the increase 

 of temperature, which causes the gaseous pressure to diminish, occasions the 

 increase of the vapour, and vice versa ; and so nearly are these two opposite 

 effects of the one cause balanced at Toronto, that the height of the barometer 

 remains very nearly the same in every month of the year; or at least, shows 

 no trace whatsoever of an annual period. 



The principle which has been thus adduced for the purpose of explaining 

 the annual and diurnal march of the atmospheric pressure should be ge- 



* The very few meteorological registers, which have been maintained with proper care for 

 several years together in Europe, are stated to afford very decided indications of the existence 

 of other fluctuations besides the annual and diurnal variations, which apparently do not 

 proceed from merely local causes, but recur regularly at stated periods of the year, and are 

 recognisable simultaneously over widely extended spaces, such for example as a considerable 

 portion of an entire continent. How far the high pressure of the month of March at To- 

 ronto may be a phaenomenon of this class it may perhaps take some years to decide. It is 

 of very marked character, and is shown decidedly in both years. As I have already remarked, 

 each year is but a single experiment in investigations of annual phainomena. 



