494 PROFESSOR FORBES ON THE DIMINUTION OF TEMPERATURE WITH 



ever, is very different in the two cases, the one being chiefly the result of the ra- 

 diation and the other of the conduction of heat. 



It is only curious that the diurnal curve seems to follow so different a law, 

 at least in summer ; — perhaps the reason is, that the direct solar radiation is more 

 energetic in that case, and the vehicular conveyance of heat by the air (or con- 

 vection) less. Thus, with respect to the process of annual heating, the earth's 

 surface (considered as an extensive plain) is the point where the sun's rays freely 

 transmitted by the atmosphere first become productive of any considerable 

 warmth. That warmth is propagated slowly and progressively by conduction to 

 the inferior strata of earth, and by convection to the superior strata of air ; in 

 either case, as I have said, a later and a feebler impress of the annual curve is 

 found. The diurnal temperature is probably much more modified by the direct 

 effects of radiation. The detached mountain tops, more exposed and less mas- 

 sive, receive and part with the solar heat more rapidly than the low country, 

 presenting a complete analogy, the former with an insular, the latter with a con- 

 tinental climate. The summits change temperature rapidly, the extremes are 

 less ; but the changes of the heat of the plain follow later, and are more marked. 

 This is not conjecture ; many facts might be quoted to support it, but the follow- 

 ing is sufficient, that Saussure, in the part of his work already cited, finds, that, 

 whilst the minimum temperature occurred at 4 a. m. (in the month of July) both 

 at the Col du Geant and at Geneva, the former station had acquired the mean 

 temperature of the day at 6 a. m., which at Geneva occurred three hours later ; 

 and, during the decline of temperature in the afternoon, the mean recurred at the 

 Col du G^ant from half an hour to an hour sooner than at Geneva. 



There are other causes besides those just mentioned which contribute to 

 distinguish the daily from the annual curve. Of these the more important are 

 the more gradual character of the annual change of temperature, and the influ- 

 ence of humidity. The former affects our experiments by preventing the ascend- 

 ing and descending currents from being instantly established, in the manner that 

 the law of specific gravity would assign ; and when radiation is least intense (as 

 in winter), and the moving power therefore small, this transfer is often impeded, 

 and even the law of densities violated altogether. This we know to be often the 

 case in winter and in cold climates, that the higher strata are the warmer. To 

 place this in a clear point of view, I shall add a table shewing the number of 

 times in each month that this has occurred, which is indicated under the column 

 headed " Number of times negative :" considering the differences of temperature 

 simultaneously observed at Bonally and Colinton as positive when the former (the 

 higher station) was colder than the latter ; and vice versa. I have also added the 

 extreme positive and negative values for each month ; and though here, more 

 than any where else, errors of observation and registration are likely to have crept 

 in, yet we cannot but be struck with the number of times in which the common 

 law of density has been reversed, and the great excess of warmth observable at 

 the higher station on some occasions, especially in autumn and winter. I would 



