320 
LIEUT.-COLONEL SYKES’S DISCUSSION OF METEOROLOGICAL 
where, speaking of the horary oscillations of the barometer, he says, “ their regularity 
is so great, that in the day-time especially, the hour may be ascertained from the 
height of the mercurial column without error, on the average of fifteen or seventeen 
minutes. In the torrid zones of the new continent, on the coasts, as well as at eleva- 
tions of neaidy 13,000 feet above the level of the sea, where the mean temperature 
falls to 44°*6, I have found the regularity of the ebb and flow of the aerial ocean un- 
disturbed by storms, hurricanes, rain and earthquakes.” It is now shown that the 
horary oscillations have a different range in different months of the year ; their range 
is influenced by height above the sea, and the tides do not always flow and ebb in 
equal periods of time; but the existence of two ascending and two descending but 
unequal tides within twenty-four hours within the tropics, is established beyond all 
question ; and if not altogether undisturbed, as Humboldt says, by storms, hurricanes 
or rain, yet the meteorological records in India prove that the periodic daily move- 
ments of the aerial ocean are never suppressed*. 
Pressure of the Atmosphere. 
With respect to the mean pressure of the atmosphere in India near the sea-level, 
I shall limit myself to the insertion of a comparative showing the means of four 
years’ hourly observations at Madras, the means of two years’ hourly observations at 
Bombay, and the observations taken every two hours (but during the day only) at 
Calcutta, for 1843-44 and 1848. I append however curves of pressure at these places 
projected for longer periods, and for which I am indebted to Dr. Buist, LL.D. The 
barometers at these respective localities, being each only a few feet above the sea- 
level, should have differed from each other in their mean annual results only in the 
third place of decimals, or at most only slightly in the second place of decimals, 
nevertheless the pressure at Bombay differs from that at Madras a twentieth of an 
inch ; and at Calcutta, where the barometer is 17 feet nearer the mean sea-level than 
at Bombay, the mean annual pressure is even less than at Bombay, while on the other 
hand the pressure at Aden reduced to the sea-level is greater than anywhere else. 
These discrepancies originate probably in the neglect of using previously compared 
instruments. At Madras, the annual means in successive years differ from each 
other only in hundredths of an inch. For the mean pressure at different elevations 
tables are annexed. 
* The observations of Lieut. R. Strachey in Thibet, at 18,400 feet above the sea, show that on the 22nd 
and 23rd of August 1849, the following were the oscillations of the four tides : — 
B. uncorrected 
for temperature. 
B. corrected 
for temperature. 
5 P.M. to 11 P.M. 
11 P.M. to 4 A.M. 
4 A.M. to 10 A.M. 
10 A.M. to 3 P.M. 
+ •059 
— •033 
+ •043 
— •037 
+ •075 
-•029 
+ •016 
-•032 
