44 REPORT — 1859. 



hood of the sea only for the pressure when the elasticity of vapour has been deducted. 

 Between these extremes of sea and continental climates a gradual passage will occur*. 



M. Dove's hypotheses (for there are more than one included in this statement) 

 were presented to the English reader first by General Sabine, in a Report on the 

 Meteorology of Toronto, published in the Reports of the British Association for 1S44, 

 p. 50, and examined by him with reference to a sea climate, that of Bombay, in the 

 Reports of the British Association for 1845, p. 73. 



In the Reports of the Association for 1845, p. 12, the Committee on Magnetical 

 and Meteorological Observations put the question, " Has M. Dove's resolution of 

 barometric fluctuation into two elements received any confirmation ? " In the 

 " Bericht," &c. of the Berlin Academy of Sciences (March 1846), M. Dove dis- 

 cusses observations made at Java, and conceives that his discussion answers the 

 question decidedly in the affirmativef- 



Mr. Broun maintained in 1846, in his discussion of the Makerstoun Observa- 

 tions, the insufficiency of M. Dove's hypothesis ; but as this has been adopted 

 lately by Sir John Herschel in a treatise on meteorology, Mr. Broun considered the 

 time was come for a careful examination of the facts on which M. Dove's method 

 professes to be founded. 



Two hypotheses are included in that method: — 1st. That the tension of vapour 

 deduced from the psychrometer observations is due to an atmosphere of vapour 

 pressing with a weight equal to that tension. 2nd. That through the action of the 

 solar heat an ascending current of air is induced ; the air is expanded and overflows 

 above over colder localities. 



In order to test the first hypothesis, Mr. Broun made some observations (in 

 January 1857) on the sea-shore of Travancore, which were compared with observa- 

 tions made in the Trevandrum Observatory eight miles distant. These observations 

 showed that the variations of the barometric pressure were to the same amount at 

 both stations, that the difference of temperature was about 0°'8 Fahr. ; nearly that 

 due to the difference of heights (about 160 feet), but that the difference of computed 

 vapour tension varied considerably; these tensions were as follows : — 



Tension of vapour. 

 19 h . 22 h . 2 h . 4#>. 9i h . 



in. in. in. in. in. 



Channavilla 0-645 0'643 0-690 0-692 0-723 



Trevandrum Q'543 Q'562 0641 Q'669 Q-68 5 



Difference 0-102 0-081 0-049 0-023 0*038 



The difference of tensions was upwards of one-tenth of an inch at 7 a.m. and less 

 than one-fourth of that quantity at 4 p.m. As this difference would have been 

 shown at the sea-shore nearest to Trevandrum (three miles distant), it was pointed out 

 that the tension of vapour thus determined, depending wholly on the different 

 temperatures of evaporation at the two stations, was quite a local phenomenon, vary- 

 ing with proximity to the source of evaporation, the temperature and the pressure 

 of the atmosphere, and the rate of diffusion of the vapour itself under such pressure. 

 On this ground the hypothesis fails completely. Indeed the diurnal variation of 

 pressure of computed dry air was shown to have the 10 a.m. maximum as well 

 marked at Chunnavilla as the barometric variation. General Sabine had obtained a 

 somewhat similar result from the Bombay observations ; and the double oscillation 

 still remaining in the dry air pressure, was explained by a supplementary hypothesis 

 depending on sea and land breezes. It was here noted by the author that the 

 Bombay Observatory was within a hundred yards of the sea, in a position quite 

 resembling that of Chunnavilla Cottage ; and that the distinctness of the double 

 maximum and minimum in the calculated dry air pressure, instead of being due to a 

 sea and land breeze, was simply due to the small diurnal range of the computed 

 vapour tension, which in the arithmetical operation of subtraction was insufficient to 

 disguise the barometric law. A few miles inland the disguise would have been more 

 marked. 



* Cited in Mr. Dove's paper " Bericht," &c. der Wiss. zu Berlin, Marz 1846, p. 54. This 

 is nearly a literal translation of M. Dove's statement, 

 t Ibid. p. 60. 



