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



ceives the addition to its vapour required to maintain an equal degree of humi- 

 dity, and the air becomes in consequence drier. This is even the case in the 

 neighbourhood of extensive lakes, as at Toronto. May is the driest and De- 

 cember the most humid month in the year: and this is also stated to be the 

 case in Europe. 



When we turn to the table in which the mean monthly tension of the 

 vapour is shown, we see most distinctly marked the connexion between the 

 temperature and the vapour pressure, and the dependence of the one upon 

 the other ; we see a simple progression, the turning points being the same as 

 those of the temperature, and a march as harmonious as we are perhaps en- 

 titled to expect from observations of only two years' continuance. 



I shall reserve what furtlier I may have to say in regard to the range of 

 the vapour-pressure in different months, until we have before us the other 

 constituent of the barometric pressure, viz. the gaseous atmosphere, to which 

 I now proceed. 



Atmospheric Pressure. 



Diurnal Variation. — The first two lines of this table exhibit the mean 

 monthly pressure on the mercurial column at Toronto at the several observa- 

 tion hours of 1840 and 1841, — the mean of the two years is shown in the third 

 line. The close accord of the mean pressure at the same hours in each of the 

 two years is a very satisfactory testimony of the confidence to which these 

 barometrical results are entitled : the mean at each hour of each year repre- 

 sents about 311 observations ; consequently in the two years the mean at each 

 observation hour represents about 622 observations, the mean of all the hours 

 in the one year 3732 observations, and in the two years 7464 observations. 



The diurnal march of the barometer may consequently be regarded as a 

 very near approximation to the truth. The diurnal march of the vapour 

 pressure is obtained by an equal number of observations, and may therefore 

 also be viewed as a very near approximation to the facts of nature. By de- 

 ducting the vapour pressure from the whole barometric pressure at each ob- 

 servation hour, we should obtain the daily march of the gaseous atmosphere. 

 This is shown in the fifth line of figures in the table ; and by taking the differ- 

 ence between the last column, (i. e. between the mean gaseous pressure at 

 all the observation hours in the two years,) and the pressure at each hour, 

 we obtain the amount by which the pressure is greater or less at each ob- 

 servation hour than the mean general pressure at all the hours. 



On first casting our eyes (in the last line of the preceding table) on this 

 representation of the diurnal variation of the gaseous atmosphere, freed from 

 the complication which its combination with the vapour pressure produces 



