Rivers, GANGES aNnD JuMNA, 101 
altitudes: and I reckon it of some consequence, to have the heights of 
these places even within 200 feet, as hitherto no idea could be formed 
on the subject. 
Wuen a tube is filled with unboiled mercury, which of course con- 
tains air, it stands at first higher than itought, from the air dilating the 
column; but, after a short time, much of the air escapes into the upper 
part of the tube, where the vacuum ought to be, and there expanding, 
presses down the mercury in the tube, thus making it lower than it 
shouldbe. ‘The mean height will not differ very much, perhaps not more 
than two tenths of an inch, in moderate heats, from that shewn by a 
boiled tube. 
‘ae barometers I had, were 2 ont of 6 sent from England, to the 
Surveyor General’s Offices they were made by Beree, and are very fine 
instruments, but so little attention had been paid to their packing, that 
the tubes of them all were found to be broken, when they arrived in 
Calcutta, as well as most of the thermometers belonging to them: there 
were spare, but unfilled tubes sent with them, and some of these would 
not fit. 
W nenever barometersare sent, there should be to each at least 6 spare 
tubes filled in England by the maker, and hermetically sealed, and these 
should be carefully packed in separate cases of copper or wood, lined 
with flannel, and the scale downwards should: go to 13 inches: the 
Cec 
