Maxima and certain Conditions of Temperature, &c. 411 



does the diurnal element, and that its explanation is a first necessary 

 step to that of the whole phenomenon. The object of the present 

 paper is to draw attention to the approximate coincidence of its 

 maximum phases with certain critical phases of temperature, cloud, 

 and rainfall, which may at least help to throw some light on its 

 physical causes. 



Forenoon Maximum. 



It was noticed independently by Espy, in 1840,* Davies, in 1859, f 

 and Kreil, in 18614 that the forenoon maximum of the barometric 

 oscillation approximately coincides with the most rapid rise of tem- 

 perature, and each of these writers attributed the rise of pressure to 

 the reactionary effect of the heated and expanding atmosphere. The 

 only data, however, given by any of them in support of the state- 

 ment are the horary variations of the temperature and pressure at 

 Prague, by Kreil, and a rough diagram of the diurnal curves at Padua, 

 by Davies; and shortly after the publication of Kreil's paper, the subject 

 was very fully discussed by Lamont, in the paper already referred 

 to in the ' Sitzungsberichte ' of the Bavarian Academy, wherein he 

 showed that, on the ordinary assumption that the atmosphere is free 

 to expand in a vertical direction, against no other resistance than the 

 static pressure of the superincumbent mass, the supposed reactionary 

 effect would be inappreciable. 



Since the publication of Lamont's pa.per, I am not aware that any 

 physicist has paid further attention to the hypothesis in question, or 

 thought it worth while to appeal to further evidence in veri6 cation 

 of the observation on which it is based, until quite recently. But in 

 1876, in noticing the subject of the barometric oscillation in the 

 ' Indian Meteorologist's Yade Mecum,' it occurred to me that Lamont's 

 assumption that the atmosphere is free to expand vertically, lifting 

 the superincumbent mass, is subject to an important modification 

 which may greatly alter the conditions of the problem as contemplated 

 by him. 



These conditions take no account of the resistance to expansion 

 that must be opposed by the highly attenuated but extremely cold 

 external atmospheric strata of great but unknown thickness, the 

 existence of which is proved by the phenomena of luminous meteors. 



If a sheet of the atmospheric envelope, of indefinite horizontal 

 extent, resting on the earth's surface, be heated and charged with 

 vapour, the first effect will be an increase of its elastic tension, which 

 will be relieved by a wave of elastic compression transmitted to the 



* ' Brit. Assoc. Eep.,' 1840, Part II, p. 55. 



f « Edinburgh Phil. Joum.,' vol. 10, 1859, p. 225. 



X ' Wien. Akad. Sitzungsber.,' vol. 43 (Abth. 2), p. 121. 



2 h 2 



