1876.] H. F. Blanford — Inequality of semi-diurnal Barometric Tides. 179 



On this hy]X)tliesis, since the two diurnal oscillations are due to differ- 

 ent kinds of action, there would be nothing even apparently anomalous in 

 the fact of their inequality. But, as a fact, the inequality of the two oscil- 

 lations is greater on the land than on the sea, greater in dry than damp 

 weather, and undergoes reversal between the plains and moiintains. As a 

 distinct featiu'e of the whole phenomenon it deserves independent study. 



Pointing out that the whole oscillation may be considered as com.- 

 pounded of a single and double oscillation, the former of which produces 

 the inequality referred to, Mr. Blanford said that in discussing the diui'nal 

 variation of the winds at Calcutta he had found that there was a diurnal 

 single oscillation of the wind-direction coinciding in the hours of change 

 &c., with the barometric single oscillation in question, and also a double 

 oscillation of the wind bearing the same relation to the double barometric 

 oscillation. Of these the first is the most important. The tendency of 

 the wind is to blow from the West (the direction of the ordinary land- 

 wind) during the day, and the opposite duiing the night. It is difficult 

 to escajDe the obvious inference that the coincidence of a westerly- wind 

 with falling pressure, and an easterly with rising pressure, both in the ease 

 of the single and double oscillation is not fortuitous. 



If the diverse effects of the sun's heat when falling on land and water 

 be investigated, it will be foimd that a greater pressiu-e will be generated 

 over the former than over the latter. A given quantity of heat used w\) in 

 the one case in heating dry air, in the other in charging it with vapour 

 without heating it, will raise the pressure of the dry air 7\ times as much 

 as that of the moist air. After allowing much for heat reflected, radiated 

 &c., it still seems highly probable that a portion of this difference vrill remain 

 outstanding, and thus will arise a diurnal inequality of pressure over land 

 and sea, a pressure, however, due to the internal motion of the air and not 

 to the quantity which exerts weight. The tendency of this will be to pro- 

 duce a transfer of air from the land to the sea in the day, and a compensa- 

 ting return current at night. 



That the amplitude of the day oscillations does depend to a great 

 extent on the kind of work done by the sun's heat is evidenced by the small 

 amplitude of the barometric curves in the rains as compared with those of the 

 dry weather, at sea as compared with land, and on cloudy days as compared 

 with clear days, which last fact was established by Kreil and Lamont. 



"Within the last few months a very elaborate summary of the data 

 recorded in different parts of the world, bearing on the subject of the baro- 

 metric tides has been published by Mr. Alexander Buchan. In re\dewing 

 these data Mr. Buchan has drawn a conclusion as to the variation of the 

 diurnal fall of pressure which at the first glance appears very paradoxical, 

 but which falls in so admirably with the conclusions just described, that it 



