1876.] H. F. Blanford — Inequality of semi-cliurnal Barometric Tides. 177 



rise takes place till about 10 at night ; but this second maximum is 

 somewhat less than that of the morning. Finally it falls, but less 

 than in the afternoon, and reaches a minimum between 3 and 4 a. m. Such 

 is the phenomenon as usually observed in Bengal, but it is subject to some 

 local variations, both as to the time of the extremes and the relative and 

 absolute amplitude of the oscillations. On hill stations 6,000 or 7,000 

 feet above the sea, the afternoon minimum is generally not quite so low as 

 the morning minimum, and the morning maximum occurs later. And, on 

 the plains, the morning maximum occurs earlier and the afternoon minimmn 

 later in the dry hot weather than in the rains ; at dry stations in the interior 

 than at damper stations near the coast. It decreases in amplitude as we 

 retreat from the tropics towards the poles, and in Europe it is always more 

 or less masked by the greater irregular oscillations to which the atmospheric 

 pressure is there subject. In the tropical Atlantic the rise and fall of both 

 oscillations are nearly equal, and apparently less than on the land. 



The phenomenon is generally spoken of as the barometric tides, but it 

 is clearly not a phenomenon of the same order as the oceanic tides, .since it 

 is quite independent of the position of the moon, and has reference not so 

 much to the position of the sun, as to the length of time he is above the 

 horizon. Atmospheric tides there undoubtedly are, similar in general char- 

 acter and origin to those of the sea, but these are not to be detected in the 

 oscillations of the barometer, except as small residual phenomena, when 

 readings are taken at different elevations and afterwards compared and 

 reduced. 



The occurrence of the diurnal oscillations and their regularity was 

 observed as long ago as the middle of the last century, and many hypothe- 

 ses have been put forward to accomit for them. 



One of the earlier explanations was that of Kaemtz who referred them to 

 the action of the smi's heat, in expanding the air and causing an overflow to 

 East and West ; while the superincumbent mass of the atmosj^here is reduced 

 to a minimum where the sun's heat falls most directly. But this wovdd fail 

 to account for the double tide, and accordingly Sabine and Dove supposed 

 that the whole iDhenomenon is composed of two distinct elements ; viz., a 

 single oscillation, which was explained on Kaemtz's hypothesis ; the result 

 of which, taken by itself would be to produce a minimum at the hottest 

 time of day and a maxinumi at the coldest ; and a double oscillation which 

 they referred to the varying tension of water vapour which has (in dry 

 covintries at least) two maxima and two minima. This view was adopted 

 by Herschell in his well known treatise, and also by Col. James in his 

 Handbook of Instructions. But it was found when tested by observation, 

 that it failed to explain the phenomenon. At Bombay, for instance, it 

 was found that when the curve of vapour tension Avas subducted from the 



