434' On the Diurnal Changes of the Atmosphere. 



the day in both places. And in the more northern or drier 

 climates, if we do not always trace the same stoppage, it is to be 

 attributed to the absence of daily cloud formation. In a very 

 dry and cold climate there is not in the course of the day suf- 

 ficient water evaporated to produce a daily thick cloud, and 

 therefore small vapour pressure goes on increasing with the 

 temperature up to the hottest period. Under these circum- 

 stances, the vapour pressure, when exhibited in a diagram, 

 forms a regular curve, having one rise and one fall in the 

 twenty-four hours ; but where much vapour exists, and much 

 more is produced daily, the dew-point does not at all times 

 indicate the pressure which results from evaporation, because 

 the rise of the dew-point is stopped at certain periods, not by 

 a cessation of the production of vapour, but through its as- 

 cent in the atmosphere and conversion into a floating cloud. 

 Boiling water in the open air does not rise above 212°, yet 

 heat continues to pass into it from the fire that is under the 

 water. The reason that the temperature of the water does 

 not rise higher is, that as much heat passes from the water 

 into the air as from the fire into the water. In like manneff| 

 evaporation of water may continue to throw vapour into the 

 air without the quantity in the air increasing, because con- 

 densation may convert vapour into water as fast as evapora- 

 tion furnishes it. But neither the fire nor the vapour is an- 

 nihilated, — the fire passes into the atmosphere and the vapour 

 becomes cloud, and we may trace both of them in their new 

 state of existence, and mark the effects they produce. 



Taking the period of a year, in all places the average daily 

 march of the temperature shows a single rise from about six 

 in the morning till one or two in the day ; and evaporation, 

 as shown by the wet-bulb thermometer, increases with the rise 

 of temperature. If the whole weight of the vapour thus pro- 

 duced were to be registered and exhibited in the form of a 

 curve, that curve would be the same in form as the curve of 

 temperature, having one rise and one fall. But in the actual 

 curve or line of the dew-point there is frequently found to be 

 a fall where there should be a rise. At Zurich and near the 

 Baltic the departure from the regular curve is considerable ; 

 in Plymouth the line is level from eleven to four; and in 

 Trevandrum, if a curve were formed, the line would cease to 

 rise at nine o'clock, five hours liefore the hottest period 1 At 

 Trevandrum the minimum and maximum of the dew-point 

 occurred within three hours, whilst on the Faulhorn they 

 were nine hours asunder ; at Zurich, in June, they were six- 

 teen hours asunder; and in other parts similar anomalies 

 occur. These irregularities may be accounted for on the sup- 



