ON THE METEOROLOGY OF PORT LOUIS. 115 



diurnal march of the total atmospheric pressure, has received from M. Dove, 

 and, after him, from General Sabine, Sir John Herschel, and others, an ex- 

 planation founded on the supposed effect of one of the constituents of the total 

 pressure, namely, the aqueous pressure. Assuming that observations of the 

 •wet and diy thermometers enable us to detennine the whole pressure of the 

 vapour in ^the atmosphere, and finding in many instances that ■when the 

 vapour-pressure thus obtained is deducted from the total pressure, the march 

 of the residual dry pressure presents a single progression, having one maxi- 

 mum and one minimum, corresponding with the coldest and hottest hours, 

 it has been inferred that the double maximum and minimum of the total 

 pressure is owing to the march of the vapour-pressure being contrary to 

 that of the gaseous pressure, au increase of temperatiure causing an increase 

 of vajjour-pressurc, but a decrease of dry pressure, and vice versa. 



Let us see whether this view will afford an explanation of the double 

 maximum and minimum of the total pressure at Mauritius. 



Diurnal Variation. — In Table XXXVIII. will be found the total atmo- 

 spheric pressure, the vapour-pressure, and the dry pressure for each hour, 

 derived from the term-day observations ; and it will be seen that the diy 

 pressure does not present a single progression, but, like the total pressure, a 

 weU-marked double progression, having two maxima at 9 a.si. and 10 p.m., 

 and two minima at 3 p.m. and 3 a.m. 



The hourly observations from which these results have been deduced were 

 not numerous, but there is little doubt that more extensive observation 

 would have led to the same conclusion ; for the six-hoirrly observations, 

 extending over a period of seven years, also give a double maximum and 

 minimi;m for the dry pressure, as wiU appear from an inspection of Table 

 XXXYII., which shows that the dry pressure has a maximum at 9| a.m. 

 and P.M., and a minimum at 3| a.m. and p.m., just like the total pressure. 



We are thus led to conclude that, if the observations of the dry and wet 

 thermometers afford the means of determining the vapour-pressure, the 

 gaseous pressure at Mauritius has a progression in every respect similar to 

 that of the total atmospheric pressure, and therefore that the phenomenon 

 in question cannot be accoimted for by the dii-ect action of the vapour- 

 pressure. 



- A similar progi-ession of the dry pressure at Bombay has been referred to 

 the relations which arise from the juxtaposition of land and sea, causing 

 land and sea bi'eezes. At Mauritius, surrounded on all sides by the Indian 

 Ocean, the double progression of the dry pressure occurs in aU kinds of 

 weather, and from whatever quarter the wind may come, and is most marked 

 on those days when the trade-wind blows steadily; and hence it is presu- 

 mable that it occurs at sea, away from the influence of land. 



Annual Variation. — At many extratrojncal stations, the annual variation 

 of the total pressure shows little trace of periodicity, but when the vapour- 

 pressure is deducted, the dry pressure is found to have a progression in 

 inverse harmony with that of the temperature. On examining Table 

 XXXIX., it will be seen that at Mauritius the annual march of the dry 

 pressiire is exactly like that of the total pressure, and that both have appa- 

 rently the same relation to the temperature. 



VI. DlEECTION AKD VeERING OF THE WiND. 



Table XXVI. shows the number of times the wind blew from the principal 

 points of the compass. The observations were taken fom- times a day during 



