Diurnal Meteorological Conditions at Kimherley. 149 



■each other, namely, that the range from the first maximum to the 

 second minimum increases with the quantity of vapour present in 

 the air. The second makes the very substantial claim that the 

 vapour has no effect whatever, excepting in the important case of 

 the second maximum. Here we have grounds for thinking that this 

 particular phase is a more or less direct result of the presence of 

 moisture, and that if this moisture were quite removed the second 

 maximum would go with it. For in the group of barometric pressures 

 pertaining to dew-points greater than the mean the night maximum 

 is some •004 inch above, and the night minimum the same below the 

 mean of the day ; whereas in the other group the same oscillations 

 would seem to sink to about one-half. Such a conclusion, at any 

 rate, is not in disagreement with the known increase in the magni- 

 tude of the night oscillation of the barometer at insular stations, 

 and at sea in the tropics. It is, on the other hand, not altogether 

 ■corroborative of Buchan's view that " the morning minimum is due 

 not to any removal of the mass of air overhead, but to a reduction of 

 the tension by a lowering of the temperature, and change of state of 

 a part of the aqueous vapour." " Certainly, since at this juncture 

 we are speaking of clear skies alone, the only possible change of 

 state would arise through the deposit of dew on the surface of the 

 ■earth. But at Kimberley even dew is not common unless it be 

 during the nights of the late summer months. Taking March, 

 April, and May together, the mean fall of pressure from midnight 

 to the first minimum is '008 inch upon very damp clear nights, and 

 •006 inch upon moderately damp clear nights. Or, leaving March 

 out, the mean fall for April and May is the same whether the nights 

 be dry or damp. (Tables 26 and 27.) 



It is worth while here to briefly note a paper by J. A. Broun ^' On 

 the Semidiurnal and Annual Variations of the Barometer" in the 

 British Association Beport for 1859. It is chiefly devoted to a criti- 

 cism of Dove's professed subduction of vapour tension from the 

 whole atmospheric pressure. He proves quite conclusively that 

 Dove's hypothesis " that the tension of vapour deduced from the 

 psychrometer observation is due to an atmosphere of vapour pressing 

 with a weight equal to that tension . . . fails completely." He 

 further remarks, inter alia : " M. Dove had brought forward as a proof 

 of the accuracy of his method, the statement that in places far in the 

 interior of the Asiatic continent, such as Catherinenburg, Nertchinsk, 

 &c., distant from large masses of water and with dry atmospheres 

 the double diurnal oscillation was not shown in the barometric 

 observations. Mr. Broun pointed out that this should not depend 

 * " Challenger " Ueport on Atmospheric Circulation. 



