1837.] 



Observations at Moulmein. 



49 



quotient arising from the division of the number expressing the tension 

 of vapour at the mean temperature of the dew-point, by that express- 

 ing the similar tension at the mean temperature of the air. The sixth 

 or last column, under the head hygrometer, affords, in connexion with 

 table II. data for calculating the amount of evaporation. 



With respect to the quantity of rain recorded in the table, it is less 

 than what usually falls during the corresponding period. The average 

 annual fall of rain at Moulmein is above two hundred inches. Formulae 

 have been given for finding the mean fall of rain at any latitude, but it 

 cannot be doubted that besides the distance of a station from the equa- 

 tor, its distance from the coast, the direction of the line of coast, the 

 relative positions of the sea and land with respect to that line, and 

 probably many other circumstances unite in influencing the amount of 

 rain. 



The times for recording the barometer were selected as furnishing 

 its two maximum and one of its minimum heights. It did not appear 

 however, that there would have been any sensible difference between 

 the mean of the observations at 9| p. m. and 10| p. m, A comparison 

 of the consecutive monthly differences of the barometer at the three 

 daily periods of observation shews that, at each of these, the column 

 of mercury decreased without intermission from January to June in- 

 clusive ; in July, it a gain increased a little. The fall of 'the barometer 

 may possibly have been caused by the shifting of the prevailing winds 

 from the N. E. and N. W. to the W., S. W. and S. as indicated by table 

 II., but there is nothing either in this, or the first table, to account for 

 the increase of pressure in July. 



"With regard to the daily oscillations of the barometer, their magni- 

 tude is undoubtedly affected by local causes, but these latter ought per- 

 haps only to be regarded as accidental disturbances of those general 

 and constant causes on which the oscillations themselves depend. It 

 appears, from the table, that both of the oscillations recorded increased 

 from January until the end of March, and then diminished: in other 

 words that though, as has been observed, the mean height of the baro- 

 meter decreased, from month to month, at each of the three periods of 

 the day, its decrease at 3i p. m. was more rapid, until the end of March, 

 than at % a. m. and 10| p. m. The oscillations at a given latitude are 

 supposed by M. Bouvard to vary in magnitude directly with the tem- 

 perature, on the centigrade scale, of the period during which they 

 occur. Is it not possible that they may also vary with the tension of 

 the atmospheric vapour ? It will be observed, on reference to the 

 table, that this tension was greater at 9§ a. m. and 9| p. m. than at 3| p. 

 m. from February to May inclusive, while on the contrary it was less 

 during January, June and July. 



