Barometric Pressure. 31 



at the surface is due to a reversal of this process as the vapor 

 settles back slowly to the ground during the afternoon and night. 

 The additional lag of the evening maximum being four hours in 

 the evening to about 10 p.m. is due to the slow cooling of the 

 ground after sunset, which continues to be a source of heat for 

 several hours, and the slow conductivity of the heated atmosphere, 

 which retains its heat even longer than the ground after the sun has 

 set. This theory, if pursued into quantitative details, will evi- 

 dently account for the entire series of observed phenomena. . . . 

 Analysing the diurnal barometric pressure by volume contents, 

 we see that with the heating of the lower strata the denser air 

 of night is replaced by contents of lower density after mid-day ; 

 taking into account the lag, the lower volumes are depleted and 

 the upper are hlled relatively, thus producing the two types of 

 periods. This is entirely analogous to the barometric pressures 

 of winter and summer, wherein the summer pressures are lower 

 [than those of winter] at the surface of the earth, but greater at 

 some such level as 1500 or 2000 meters, the summer pressure 

 corresponding to that of the diurnal pressure in the afternoon. 



It is inferred from these considerations that since the 



double diurnal period is confined to a thin sheet near the surface 

 and does not extend throughout the atmosphere. Lord Kelvin's 

 theory of a dynamic forced wave is not available for explaining 

 this phenomenon." 



There seem to me — I hope you will agree that I am speaking 

 with all diffidence as one seeking rather than imparting information — 

 there seem to me great difficulties in Prof. Bigelow's exposition. In 

 the absence of direct proof it is surely difficult to believe that there 

 can be such an enormous vertical circulation of air extending from 

 the bottom to an altitude of upwards of 10,000 feet as seems to be 

 implied. Then there is the circumstance that over the ocean the 

 vapour tension varies with the temperature and has only a single 

 maximum and minimum in the day, whereas the pressure of the air 

 has a semi-diurnal period as strongly pronounced as it is over the 

 land, in spite of the almost uniform temperature of the lower air and 

 of the floor upon which it rests. 



Now, I have troubled you at so much length with these various 

 theories because each one seems to me to contain something worth 

 attention. Certainly they all display a certain amount of irresponsible 

 slurring over difficulties, and base their strongest pretensions on what 

 we know least about ; nevertheless they are not to be entirely rejected 

 on that account. They compare very favourably, at any rate, with 

 some of the theories that have attained transient currency in other 

 sciences. 



The most important contribution so far made to the theory of 

 the diurnal oscillation of barometric pressure is due to Hann. 

 This renowned meteorologist has done a vast amount of work in 

 classifying and generalising the harmonic elements of the pressure 

 variations for a great number of stations, and has succeeded in 

 establishing a number of re.=;ults of the first im])ortance. Harmonic 



