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LIEUT.-COLONEL SYKES’S DISCUSSION OF METEOROLOGICAL 
the minimum daily mean depression at the end of November. From the means 
of eight years’ observations, obligingly furnished to me by Mr. Glaisher, from the 
Royal Observatory, Greenwich, the maximum was in October ; with a second maxi- 
mum in December, the minimum was in April ; corrected however for the tension 
of vapour, the maximum was in December and the minimum in August; but conse- 
cutive years differ greatly when the vapour correction is applied. 
I attempt no explanation of these anomalies, as much more lengthened observa- 
tions are required than those at my disposal afford for philosophical deductions. 
From the month of the maximum pressure, there is a gradual mean monthly decline 
of pressure until the minimum pressure; then there is a gradual monthly increase 
of pressure, even when the monsoon sets in at Madras in October until the maximum 
is attained again ; but there is no rule without an exception, and the curve was inter- 
rupted in the month of December in the years 1844 and 1845 at Madras, and in 1844 
at Bombay; but in Calcutta, in October 1844, and at Mahabuleshwur, and at Doda- 
betta in December 1828 and 1847 respectively. At Aden, in 1847, it was interrupted 
in February. In 1848 the curve was regular. 
Appended to this paper are annual pressure curves at Madras, Bombay and 
Calcutta, protracted from the records of the barometer at the several places. The 
three localities are under very different conditions of temperature and moisture in 
the same months, and they differ considerably in latitude and longitude ; Madras and 
Bombay, situated upon opposite sides of the peninsula of India, are subject to different 
monsoons ; Madras to the N.E. monsoon, which commences in October and ends in 
February, and Bombay is subject to the S.W. monsoon, which commences in June and 
ends in October. Calcutta is under the full influence of the S.W. monsoon, but has 
occasional showers from the Madras rains. When Bombay is deluged with rain 
Madras is comparatively dry, and the hot weather prevails ; and when Madras is 
under the influence of the Coromandel rains, Bombay is cold and dry. Very different 
atmospheric conditions therefore exist at the two places in the same months. Never- 
theless the annual curves of pressure protracted from monthly means, may be said to 
be identical, not only for Madras and Bombay, but also for Calcutta. The few ex- 
ceptional cases are lost or disappear in the means, and may originate in local causes. 
It would thus appear that the periodic movements of the mass of the atmosphere 
within the northern tropic are independent of, or only very slightly affected by, the 
hygrornetric and thermometric conditions of the lower strata of the atmosphere. It 
occurs to me that this phenomenon may be owing to the sun’s place in the ecliptic. 
When the sun is at the southern tropic, the air above the northern tropic is compa- 
ratively cold and therefore dense, and at its greatest pressure ; this would be from 
December to January inclusive. As the sun returns to the north the air gets gradually 
warmer and dilates, and the pressure being inversely as the volume, the barometer 
gradually sinks, until the sun is returning from the northern tropic again, when the 
mass of the atmosphere gradually cools, gets denser, and the pressure gradually in- 
creases again. Supposing this explanation to have any foundation in truth, the curve of 
pressure within the tropic south of the equator would be the reverse of that in the tropic 
north of the equator. The maximum pressure would be from June to July inclusive. 
