316 Dr. Bostock on Evaporation. 



The greatest quantity of evaporation, in one hour, is 1-75 gr. ; it 

 occurred on August 4th, 1813 ; the least quantity of evaporation 

 ■was on the 12th of November, 1812, when no loss of weight could 

 be perceived : on the 1 st of December in the same year, it ap- 

 peared that there was even some increase of weight. The greatest 

 winter evaporation was on the 28th of November, 1812, amounting 

 to 1-08 gr., and the least summer evaporation on August 5th, 1813, 

 amounting to -25 gr. per hour. 



The average of fourteen winter observations, with the wind in a 

 S. or W. point, is '346 gr. per hour ; of fourteen summer observa- 

 tions, with a S. or W. wind, is -882 gr. ; of fourteen winter observa- 

 tions, with the wind in a N. or E. point, is -546 gr. per hour; of four- 

 teen summer observations, with a N. or E. wind, is I'OSgr. per 

 hour. 



In order to observe whether the rate of evaporation was affected 

 by the height of the barometer, the barometric scale from 29-20 to 

 30'20, was divided into ten equal parts, and the same number of 

 observations being taken under each division, the respective quan- 

 tities of water evaporated per hour were as follows: -381, -451, 

 •436, -386, -76, -75, -51, -545, -565, and -471, gr. If we are to 

 consider the relation which these numbers bear to each other as 

 any thing more than accidental, it would appear that the state of 

 the atmosphere which is attended by either a very low or a very 

 liigh barometer, is less favourable to evaporation than the interme- 

 diate state. Nor does it seem unreasonable that this should be the 

 case. Damp or wet weather, which is generally accompanied by a 

 low barometer, is obviously unfavourable to evaporation ; while, 

 when the barometer is high, the atmosphere may be supposed to be 

 more nearly saturated with moisture, and, therefore, less disposed 

 to receive an additional quantity. According to the preceding ob- 

 servations the interval between 29-60 and 29-80 would appear to 

 be the most favourable to evaporation, while the range above 29*80 

 is more favourable to it than below 29-60. I could not perceive 

 that the condition of the barometer, with respect to its being in the 

 rising or falling state, independent of its absolute height, bore any 

 relation to the amount of evaporation. 



