74 



Report S.A.A. Advancement of Science. 



Confining our attention for the moment to the chief term and to 

 fix our ideas, let us suppose that it is due to the Sun's action, — that 

 as summer comes on the air is expanded by the solar heating of 

 the ground or sea and pressure thereby diminished and that as winter 

 comes on the opposite effect takes place. On such an hypothesis, it 

 is evident that the ground ought to get warm more quickly in spring 

 than the sea and cool more quickly in autumn. This would be a 

 ground effect and to a certain extent independent of altitude as long 

 as the ground was level or plateau-shaped, but that at quite moderate 

 elevations on isolated mountains, and to a lesser extent on mountain 

 ranges, the local heating should have no effect as there is an escape 

 on all sides for the heated air. It will be seen that the figures 

 conform very closely to this hypothesis. At ocean or coast stations the 

 angle is always smaller than at inland stations : — 



Coast 



Inland. 



These figures show that the extreme readings at continental stations 

 occur nearly a month earlier than at the coast. 



The decrease of amplitude on mountain stations is shown by the 

 following examples : — 



Cape . 



Devil's Peak 

 Disa Head 



Tamatave 

 Tananarive 



Umtata 

 Kilrush 



Daressalam 

 Kwai 



0.1 13 inches 



0.081 



0.063 



0.160 

 0.089 



o.ioi 

 0.060 



o. 109 

 0.060 



37 

 1,436 

 2,496 



10 



4,500 



2,400 

 6,850 



40 

 5,280 



feet alt. 



Johannesburg is on the summit or ridge of a range which on each side 

 falls away some 1500 feet, hence perhaps its small amplitude. 



On the hypothesis, the time of highest and lowest mean pressures 

 have to be reversed north of the menn thermal equator, so that the 

 angle 270° ought to become 90°. To show that this is so, the table 

 includes the following normal places, — Perim, Aden and India 30° N 

 as well as some others. It is evident that in going from S to N we 

 should meet with a neutral zone where the annual variation of pressure 

 is almost or entirely lost. It would appear that on the W coast this 



