122 REPORT— 1867. 



the 13th and 20th of April. The wind remained at S.E. to E.S.E. and E. 

 throughout, and in the gusts blew with a force varying from 1 to 13'5 lbs on 

 the square foot. The barometer ranged from 30"174 to 29-944, and oscillated 

 during the gusts. Very little rain fell. On the 21st the wind veered to the 

 north of east and fell light. It was afterwards ascertained that several re- 

 volving storms occui'red from the 6th to the 25th of April, between the inner 

 borders of the monsoon and trade-wind, away to the northward and north- 

 eastward of Mauritius. 



This is not the time to enter into a discussion regarding the nature and 

 origin of these storms : I wiU only remark that, by watching the barometer, 

 the wind, and the clouds, their existence and approach may be known with 

 certainty, even when the distance is very considerable. 



XII. Synopsis of Eestjlts. 



With a view of facilitating a comparison of the results, I have prejoared 

 a few Tables in which the diurnal, monthly, and annual means of the prin- 

 cijial elements are placed side by side. 



Diurnal Variation.- — Table XXXVII. exhibits the means for each observa- 

 tion horn' of the six-hourly series, derived from seven years' observation ; 

 and Table XXXVIII. those for each hour of the day, derived from term-day 

 observations taken for foiu* years. As akeady remarked, the diurnal march 

 of the temperature, vapour-pressure, force of wind, and amount of cloud are 

 all more or less accordant, being in the same sense, and having the turniug- 

 points nearly at the same hours. The diurnal march of the humidity is in a 

 contrary sense, but the turning-points are nearly coincident with those of 

 the temperature. With regard to the total atmospheric pressui'e, and the 

 pressure of the dry aii', they have a double progression, with four turning- 

 points. 



Amiual Variation. — Table XXXIX. exhibits the monthly means. The 

 temperature in the shade and in the sun's rays decreases from January to 

 July, and then increases from July to January. The atmospheric pressure 

 increases from February to August, and then decreases from August to Feb- 

 ruary ; and the march of the dry pressure is similar. The vapour-pressure 

 has a progression in direct agreement with that of the temperature, showing, 

 however, a tendency to a second maximum in August. With respect to the hu- 

 midity, we see that it has a double progression, with two maxima in February 

 and August, and two minima in June and November. The mean monthly 

 force of the wind also has, upon the whole, a double i)rogression, having 

 two maxima in February and June, and two minima in April and November. 

 The rainfoU, too, has a double progression, with two maxima in February 

 and August, and two minima in June and September. The amount of cloud 

 has a maximum in February and a minimum in June, with a tendency to a 

 second maximum in August. A similar remark applies to the frequency of 

 lightning. 



Table XL. gives the means of the extreme monthly range of the principal 

 elements. The temperature and humidity are, on the whole, subject to greater 

 fluctuations in the summer than in the winter months, and the greatest fluc- 

 tuations of the vajiour-pressure take place from January to June inclusive. 

 A comparison of the oscillations of the total atmospheric pressure and va- 

 pour-pressure will show the important part played by the latter. 



Secular Variation. — Table XLI. exhibits the extreme annual range, and 

 Table XLII. the annual means of the several elements for each year. 



