ON THE METEOROLOGY OF PORT LOUIS. Ill 



The last line in Table VIII. shows that the temperature in the sun's rays 

 was cousiderably greater in 1860 than in any other year, and that, upon the 

 whole, it decreased till 1864, and has been increasiag since that year. 



Extreme Annual Range. — The extreme annual range of temperature for 

 each year is given in Table X., containing the highest and lowest readings 

 of the self-registering thermometers, and the epochs of occui-rence. The 

 mean annual range is 22°- 52. 



II. Elastic Poece op Vapour. 



The pressure of the atmosphere, as measiu-ed by the barometer, is the 

 combined pressures of the dry air and the aqueous vapour suspended in it ; 

 and many are of opinion that, by means of simvdtaneous observations of the 

 barometer and dry and wet thermometers, the two pressures may be sepa- 

 rated and exhibited apart. 



Diurnal Variation. — The last line but one in Table XI. gives the diurnal 

 march of the vapour-pressiu-e in inches of mercury, as deduced by Glaisher's 

 Tables from the six-hourly observations of the dry and wet thermometers. 

 "We perceive that the pressure is greatest (-6.58) at the warmest observation 

 hour (3| P.M.), and least (-646) at the coldest hour (3| a.m.), which is what 

 would be expected, since the' capacity of air for vap'our is directly as the 

 temperature. The last line shows the amount of variation, which is shght. 



The last two columns in Table XII. exhibit the mean vapour-pressure for 

 each hour of the day, and its deviation from the daily mean, obtained from 

 the houi-ly term-day observations for four years (1863-66). Here we have 

 complete evidence of a direct harmony between the diurnal march of the 

 vapour-pressure and that of the temperature, the hours of the greatest vapour- 

 pressure coinciding with those of the greatest temperature, and vice versa. We 

 see that the march of the vapour-pressure, though a little irregular, is like the 

 march of the temperature, a single progression, havmg two brauches, the one, 

 upon the whole, ascending from 4 a.m., when the pressui-e is least (-621), to 

 1 P.M., when it is greatest (-646), and the other descending from 2 p.m. to 

 4 A.M. Between 6 and 8 a.m., as the heat increases, the pressure takes a start 

 upwards, and from 8 a.m. to 3 p.m. it is nearly stationary. From 3 to 4 p.m., 

 as the heat declines, the vapour-pressru-e also declines, and again continues 

 nearly uniform till 2 a.m., between Avhich hour and 4 a.m. it falls once more. 

 From 8 a.m. to 5 p.m. it is above the mean for the day, and from 5 p.m. to 

 8 A.M. below it, attaining its mean value about 7 a.m. and 5 p.m. 



Anniwl Variation. — The last two cohunns in Table XIII. give the annual 

 march of the vapour-pressure, and the amount of its deviation from the 

 annual mean, derived from the six-hourly observations. Here also we have, 

 upon th^ whole, a single progression. The vapour-pressure attains its maxi- 

 mum (-767) in February, and its minimum (-550) in July. From February to 

 July it decreases, and from July to February it increases, except in September, 

 when it is less than in August,— August, as m'c shall presently see, being a 

 month in which not only the vapom'-pressure, but also the humidity, rain- 

 fall, and cloud are greater than in the months immediately preceding and 

 following it, these elements showing a tendency to a small second maximum. 

 During the six summer months the vapour-pressure exceeds the mean for 

 the year (-652), and during the six winter months it falls short of it. There 

 is thus a connexion between the annual variation of the vapour-pressure 

 and that of the temperature of a kind similar to that between the diurnal 

 variations of the same elements, the progressions being in the same dii-ec- 

 tion, and the turning-points nearly coincident. 



