Semi-diurnal Fluctuations of the Barometer. 173 



gether, are adequate to the production of the fall of the baro- 

 meter from 10 to 4 o'clock in the day. 



And if we proceed with our inquiries into the next period 

 of six hours, that is, from 4 to 10 p.m., we meet with facts 

 that do not harmonize with the temperature theory. During 

 the whole of this time, it is true the temperature falls and the 

 barometer rises: but the vapour pressure must have dimi- 

 nished according to the temperature theory, as the dew-point, 

 the measure of vapour pressure, falls ; and the lowering of the 

 dew-point after 4 o'clock showed that vapour was then con- 

 densing in the lower part of the atmosphere. So that here it 

 becomes necessary to suppose that the atmosphere cools 

 enough, not only to raise the barometer to the full extent of 

 its daily range, but also to counteract the reduction which takes 

 place at the same time in the vapour pressure. Again, from 

 10 at night, although the atmosphere continued to cool, the 

 barometer did not continue to rise, but once more fell, which 

 fall is attributed to a diminution of vapour pressure. Thus 

 from 4 to 10 in the afternoon and evening, cooling the atmo- 

 sphere is represented as more powerful than reduction of va- 

 pour pressure ; and from 10 in the evening to 4 in the morn- 

 ing, reduction of vapour pressure is supposed to be more pow- 

 erful than cooling the atmosphere. The two forces, we are 

 required to believe, do not merely neutralize each other, but 

 each in its turn exercises a paramount influence, and for the 

 time determines an absolute rise or a fall of the barometer; 

 and this we are called upon to admit without any satisfactory 

 or even plausible evidence being adduced to prove it. 



What has been here advanced applies with the greatest 

 force to the semi-diurnal fluctuations in atmospheric pressure 

 which take place within the tropics. Aqueous vapour exists 

 in the atmosphere in larger proportions in that part of the 

 world than it does in higher latitudes ; and it is to the daily 

 condensation of that vapour in the atmosphere, and its subse- 

 quent evaporation there, that we are really to attribute the 

 great deviation of the movements of atmospheric pressure from 

 the daily march of temperature. If no vapour existed in the 

 atmosphere, the alteration of pressure would be very little, 

 and it would be the reverse of temperature. As the atmo- 

 sphere became warmer, the pressure would be less ; as it be- 

 came colder, the pressure would be more. And the hourly 

 variation in the quantities of vapour actually found in the at- 

 mosphere which arises from alteration of surface temperature, 

 only introduces another element of pressure into the inquiry, 

 which is simple in its character, — the vapour increasing or 

 diminishing with an increase or diminution of temperature. 



