162 ALLISON FLUCTUATION OF THE BAROMETER. 



fact is again shewn by the deposit of dew being greatest in the early 

 hours of the evening, between sundown and 9 p. m., when the 

 humid vapour of the atmosphere descends most rapidly and forms 

 in drops of visible water upon the ground and other material sub- 

 stances as they cool. The most energetic force then being at an 

 end, the mercury about 9£ p. m., again gradually retires down the 

 tube till 3 a. m., or a little after. But why should this second 

 descent not continue, at least till the rising sun rarifies the atmos- 

 phere by heat, when the downward tendency would be accelerated, 

 as we have seen that it is after 9 a. m. ? Because as daylight 

 approaches, and the earth is at its coldest, the expansion of the 

 vapour, which we have noted as condensing and pushing downwards 

 at the beginning of night, re-commences. The earth which- parts 

 with heat less readily than does the atmosphere, is also more loth 

 to take it in, and now about 3 J in the morning (as an average hour 

 for the twelve months) is most cold. Then the layer of atmos- 

 phere immediately on the earth's surface is the next deficient of heat, 

 and the higher strata (I speak within limits proximate to our planet) 

 are the warmer. So the night vapour rises, but not only does it 

 rise, but with its " quasi explosive force," as says Sir Henry James 

 in commenting upon Professor James Espy's interpretation of this 

 phenomenon, it "presses upon the delicate barometer and the 

 mercury rises, till our starting point of 9 a. m. is again reached, 

 when the atmosphere being heated and dried and the ground 

 warmed, the fluctuation again begins its diurnal career. This 

 motion is known .to be constant in regular weather, so much so as 

 to be spoken of as " Diurnal Atmospheric Tide " ; but while proved 

 to take place on all the continents (which it will be remembered are 

 chiefly inhabited on their shores, or at comparatively small distances 

 from the oceans) it becomes very obscure in the interiors of large 

 land tracts. This is well marked in our own country. Even in 

 Ontario these tides are not so great as here beside the sea. In 

 Manitoba still less. And when we have got a sufficient number of 

 observations from that district, between Winnipeg and the Rocky 

 Mountains, I shall not be surprised to find the double fluctuation 

 almost obliterated ; and but one well defined maximum and mini- 



