210 Prof. Forbes on the Diminution of Temperature 



rapid increase of the Difference of Temperature between Fe- 

 bruary and March, and a most rapid decUne in November, the 

 maximum being about May. 



That the decrement of temperature with height is most ra- 

 pid in summer, and least in winter, has been long known ;* but 

 I am not aware of any attempt to account for the law of its 

 variation at different seasons. The following considerations 

 will probably be found satisfactory. 



If we* examine the annual curves of mean temperature at 

 Colinton and Bonally, projected in the lower part of Plate 11. , 

 we shall find that they differ in three respects, whilst there is 

 a remarkable coincidence in their general features. (1.) The 

 entire Bonally curve stands lower on the paper than the Colinton 

 curve, because the mean temperature of any and every part of 

 the year is lower. (2.) It is a flatter curve than the Colinton 

 curve, or the range of the thermometer is less ; consequently the 

 minima differ less than if the two curves had been similar, and 

 the maxima differ more. This is the reason why the decre- 

 ment of temperature with height is most rapid in summer, and 

 least so in winter. (3.) Not only is the Bonally curve lower 

 than the Colinton one, and flatter, but it is shifted to the right 

 hand, so that the maxima occur later, as well as the minima 

 and mean temperatures. A little attention will likewise shew 

 that a gap or vacuity must be left between the curves, greatest 

 whilst the temperature rises, and least whilst it falls ; and also 

 that the difference of the vertical ordinates of the two curves 

 will be greatest when they form the greatest ascending angle 

 with the horizontal axis, and least when the descending angle 

 is greatest, that is, as inspection shews, in May and Novem- 

 ber respectively, which agrees with the results of the upper- 

 most curve of the plate. 



The examination of these curves furnishes us with some data 

 of the most important kind for meteorology, which it is best 

 in the first place to state, and afterwards to consider how we 

 can explain. 



The first fact is the familiar one, that mean temperature di- 

 minishes as we ascend in the atmosphere. The second is, that 

 ii .1 — ■ ■ 



* See my Report on Meteorology in the fii-st voluhie of the British Asso- 

 " tifttion Reports. 



