Semi-diurnal Fluctuations of the Barometer. 175 



pour into dew, when the atmosphere becomes somewhat lighter 

 up to about 4 or 5 in the morning. 



As we proceed from the equator towards higher latitudes, 

 we find less vapour in the atmosphere, and its influence on 

 atmospheric pressure is less marked. At Padua the fall of 

 the barometer from 10 to 4 in the day is not much more than 

 one-fourth the extent that it is at the equator, and at St. 

 Petersburg it is very small. In situations where there is not 

 sufficient vapour in the atmosphere to form any daily cloud, 

 it is to be presumed that if a barometrical registration were 

 to be made, there would be no double movement exhibited 

 showing a fall from 10 a.m. to 4- p.m., and a rise from 4 to 

 10 p.m., because there would be no condensation and warm- 

 ing to produce the former, nor evaporation and cooling to 

 cause the latter. 



The heating effects of condensing vapours may however be 

 traced even in comparatively dry latitudes, such as that of 

 Toronto, as shown in Col. Sabine's report to the British As- 

 sociation in 1844. There was no fall of the barometer at that 

 place from 4 to 10 in the morning, although the temperature 

 had risen from 39°'20 to 46°*35, above 7°; but in the middle 

 of the day, from 1 to 4, with an increase of temperature from 

 46 0, 35 to 50 o, 55, being only 4°'20, the gaseous as well as the 

 general atmospheric pressure was materially reduced ! not- 

 withstanding that the increase in the quantity of vapour du- 

 ring this time must have been as great as it was in the pre- 

 ceding period ; and if this increased quantity had remained in 

 the atmosphere, its pressure must have been added to that 

 which previously existed. We are then obliged to suppose 

 that the reduction of the pressure which took place imme- 

 diately after 10 o'clock, arose from a cause which came into 

 operation at that time; and that cause it is contended can be 

 found only in the heating of the atmosphere by the conden- 

 sation of vapour. 



The great defect of the temperature theory is, that it fails 

 to account for the fall of the barometer from 10 a.m. to 4 p.m., 

 and its subsequent rise from 4 to 10 p.m., though this is the 

 oscillation for which we have particularly to account; whilst 

 the theory here maintained points out the cause of these, as 

 well as the other diurnal, and also of the casual movements of 

 the barometer. We are therefore at liberty to conclude that 

 the semi-diurnal fluctuations of the barometer can be accounted 

 for only on the condensation theory. 



