468 
Proceedings of the Royal Society 
the height for all seasons, the results from observations made in 
January will differ from 250 to 300 feet from those obtained from 
observations made in July at the same place. If no account he 
taken of the daily variation of the pressure, the observations made 
in July at 9 a.m. will give a difference of from 350 to 400 feet in 
height as compared with results from observations made in July at 
4 p.m. All this large error is avoided when the monthly and the 
daily variations are allowed for. 
It has been seen that the summer pressure in Central Asia falls 
in July to about 29*500 inches. It might be inferred by analogy 
that the pressure in Central Africa also falls considerably below 
29*92 inches over those regions where the sun is nearly vertical; and, 
as a consequence, that this space of low pressure moves north and 
south with the sun, attaining its northern limit in July, and its 
southern in January. The figures in the table given above fully 
bear out this supposition. Thus, in July at Algiers the mean 
pressure is 30*06 ; but at Laghouat, between 280 and 300 miles 
inland, the pressure is only about 29*86 inches ; at Alexandria it is 
29*80 ; and at Aden, only 29*69 inches ; and since, in the same 
month, according to Speke, the wind in Central Africa near the 
equator and long. 32° 20' E. is almost constantly S.E., it is pro- 
bable that the pressure there is lower than at Aden. Taking the 
whole facts into consideration, it can scarcely be less than 29*70 
inches, though probably it is lower. Again, in January the pres- 
sure at Cape Town being 29*97 inches, at G-raff Reinet 29*91 inches, 
and at Maritzburg 29*89 inches, points still further to a diminu- 
tion of pressure in the centre of southern Africa at this season, in- 
creasing from the coast — falling, probably to between 29*70 and 
29*80 inches. Hence, if we assume 29*70 inches as the low pres- 
sure which accompanies the sun over those parts of Africa where 
he is nearly vertical, we shall not be far from the truth. 
Let us apply this reasoning to the determination of the height 
of Albert Nyanza from Sir Samuel W. Baker’s observation of the 
boiling point of water. The observation was made in lat. 1° 14' N., 
long. 30° 50' E, on 14th March 1864, between 8 and 10 a.m., pro- 
bably at 9 a.m. The boiling point of the thermometer was 207°*8, 
but as it changed while in Sir Samuel Baker’s possession, it is 
supposed that the true reading was about 207°*3, which corresponds 
