Barometric Pressure. 29 



regions adjoining, owing to the inertia and viscosity of the air, 

 pressure continues to fall to the morning minimum. This 

 minimum is thus due, not to the removal of any of the mass of 

 air overhead, as happens in the case of the afternoon minimum, 

 but to a reduction of the tension or pressure of the air consequent 

 upon a reduction in the temperature through radiation from the 

 aerial molecules towards the cold regions of space." 



This differs somewhat (and for the better, I think) from Buchan's 

 explanation given in the CJiallenger Report, in which the morning 

 maximum is said to depend in part upon the " vitally important 



principle that a portion of the aqueous vapour of the 



atmosphere passes from the gaseous to the liquid state, thus reducing 

 the tension," and an inverse process as a contributing cause of the 

 morning maximum. 



The great virtue of Buchan's theory, to my mind, lies in his 

 delinite acceptance of the principle that a great portion of the 

 barometric oscillation may be due to tension as distinguished from 

 lateral movement, and rejection of the principle that the diurnal 

 oscillation differs only in degree from the annual circulation. Indeed, 

 the weak spot in his theory is in the hypothetical nature of these same 

 lateral movements, and for which he adduces next to no evidence. 

 The high velocities that would be requisite if an air particle is to move 

 all the way from m^ to Mb — about a thousand miles an hour at the 

 equator — seem to have escaped attention ; and also the complicated 

 nature of the circulation necessary to restore equilibrium. For the 

 movement in the first instance will not be a simple flow from W'here 

 the atmospheric strata are most elevated to where they ftre most 

 depressed ; but will (or should) operate most vigorously where the 

 gradients are steepest. That is, any outward tendency of the wind due 

 to an elevation of the atmospheric strata should display its most 

 marked effect near the times of sunrise and sunset, when temperatures 

 are changing most rapidly, and should be directed to some point not 

 the shortest path down the slope. As we have seen, this is pretty 

 much what happens at Kimberley, and, as I have shewn elsewhere, 

 our normal wind directions at any hour are tangents to an imaginary 

 spiral curling outwards from the light hemisphere and into the dark. 

 Another point which seems to wnnt demonstration, or, at any rate, 

 elucidation, is the local character he endeavours to give to his theory, 

 namely, that the barometric oscillations are generated by absorption 

 and radiation in the regions where they occur. It is difficult to believe 

 that physical processes which can only generate a total range of tem- 

 perature over the ocean of one or two degrees can give rise to a range 

 of pressure often greater than it is in continental places, where the 

 range of temperature is twenty times as great. Moreover, it has yet to 

 be shewn that the absolute humidity (upon which absorption depends), 

 and the relative humiditv (upon which radiation mainly depends), 

 are materially less at great altitudes over the land than they are over 

 the ocean. Over Kimberley, at any rate, the upper air must be nearly 

 or quite as rich in aqueous vapour as the upper air is over the eastern 

 Atlantic. For our upper currents seem to set almost constantly from 



