46 Extracts from the “ Journal of a Sw'vey to explore (Jury, 
Thermometer attached ....... Seat art iin 
Ditto detached. .........- BS ic rs 68° 
Having hung the barometer up in the tent, and allowed it to 
acquire the temperature of the air and adjusted zero, the follow- 
ing heights were observed : 
Thermometer attached ......0+0...0+6 774° 
Ditto detached ..... 20.26.60. epee «3 DOr 
Upper surface of the mercury......... 20 in. 
Second reading an hour afterwards, 
mercury upper convex surface...... 20 
An hour afterwards upper convex. .... 20 
Afternoon, outside of the tent three 
hours after filling the tube ; mean at 
four o°clogk... «is:sists))cistenn'h 0a eieininotelene 
There were very few and but small (air) bubbles in the column, 
and the vacuum was evidently pretty good, as shown by the 
smart cracking of the mercury against the top of the tube. 
Wiateriboiletatss..sinetcearcie elon ve 196° 
We now begin to boil the mercury in the tube. The tube as 
usual broke. None but a professed artist can expect to succeed 
in this difficult business once in 10 times. With the unboiled 
mercury, there must be an error, but it should not, I think, 
affect the heights more than 200 feet, and generally not 100 feet; 
and as under the present circumstances we cannot do more, we 
must be content with such approximate 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. 
When a tube is filled with unboiled mercury, which of course 
eontains air, it stands at first higher than it ought 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 Jower than it should be. The mean height will 
not differ much, perhaps not more than two-tenths of an inch in 
moderate heats from that shown by a boiled tube. 
The barometers I had were two out of six sent from England 
to the Surveyor-General’s Office. They were made by Berge, 
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. 
Whenever barometers are sent, there should be to each at 
least six spare tubes filled in England by the maker, and herme-~ 
