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Toronto, in Canada, for 1846 and following years, at the 
government establishment in that place. The hourly heights 
from four in the morning till ten at night, of the barometer, 
and the dry and wet thermometers, together with the tension 
of aqueous vapour, were given from the registers for the 
month of July, 1846, in order to trace the cause of the daily 
alterations of atmospheric pressure. The barometer being 
presumed at all times to give the total pressure, the portion 
of it arising from weight of vapour is shown by its tension, 
as ascertained from the dew-point ; and this being subtracted 
from the total pressure, leaves the remainder to express the 
weight of the dry gases, which weight, it is considered, varies 
in the open atmosphere with their temperature. But it was 
shown from the recorded numbers, that the wet and the dry 
separate elements of pressure did not agree with the total, 
more particularly from nine in the morning until five in the 
afternoon, during which time the barometer falls. And it 
was asserted that the amount of tension did not present 
evidence of the quantities of vapour that were sent into the 
atmosphere at the time by the process of evaporation ; as 
while the wet thermometer showed that much more vapour 
was passing into the air, its tension became considerably less. 
The temperature of the air also, which indicates variation in 
the weight of the gases, was shown not to accord with the 
movements of the barometer from four a.m. to four p.m. 
The conclusion arrived at from these various facts was, that 
temperature at the surface, as shown by the thermometer, 
and tension of vapour, do not, as commonly supposed, give 
truly the general atmospheric pressure ! It was also pointed 
out that, during the first six hours, from four to ten, vapour 
raised the barometer, and yet a large increase in its quantity 
afterwards was attended by a great fall ! From these cir- 
cumstances it was concluded that tension of vapour, and 
temperature at the surface, do not account for the changes 
of atmospheric pressure. The action of vapour was then 
