467 
of Edinburgh , Session 1868 - 69 . 
difference of the two observations, each being here supposed to be 
taken under the most favourable circumstances, and with the greatest 
accuracy, being 680 feet.* Observations made in the first half of 
April, or in the latter half of October, when the pressure is the mean 
of the year, supply the best data for the calculation of heights. 
If the best physical atlases be examined, and the heights, 
given by different authorities, of table-lands and mountains, of 
Central Asia, Central Africa, and the highlands of the United 
States and British America be compared, considerable confusion 
will be found to prevail. 
One or two examples may be given to show the application of all 
this. From barometric observations made on the 28th November 
1838, the level of the Dead Sea below that of the Mediterranean 
was calculated to be 1429 feet. The real depth of this sea below 
the level of the Mediterranean, as determined by the English 
engineers by levelling, is 1296 feet. Now, since the mean 
pressure of the atmosphere over the region of the Dead Sea in the 
end of November is about 30*035 inches, it is seen, if the sea-level 
pressure was assumed to be 29*9 inches, how the lake came to be 
lowered 133 feet. 
Much interest is at present attached to the heights of Central 
Africa. The following mean pressures at 32° and sea-level bear on 
this interesting question : — 
January. 
July. 
January. 
July. 
inches. 
inches. 
inches. 
inches. 
Malta 
30-07 
30-01 
Cape Town 
.... 29-97 
30-20 
Algiers 
3015 
30-06 
Graff Reinet.... 
.... 29-91 
30-22 
Laghouat ( Algeria) 
30-07 
29 86 
Maritzburg 
.... 29-89 
30-19 
Gibraltar 
30-18 
3006 
Mauritius 
.... 29-95 
30-19 
Christiansborg 
29-92 
30-04 
Aden... 
.... 30-03 
29-69 
St Helena 
30-05 
30-18 
Alexandria 
.... 30-06 
29-80 
Grahamstown 
29-91 
30-15 
Thus the difference at G-raff Reinet and Maritzburg between the 
January and July pressures amounts to about 0*30 inches. From 
this it may be inferred that, in calculating heights along the 
Zambesi, from observations made at different seasons, if no allowance 
be made for the monthly variation, but if 29*92 inches be assumed as 
* The height of Lake Balkash, according to the Russian explorers Ssemonoff 
and Golubeft', may be anywhere between 530 feet as given by the former, and 
1200 feet as given by the latter. For a large number of heights made use of in 
writing this note, the author is indebted to Mr Keith Johnston, jun. 
