490 



PROFESSOR FORBES ON THE DIMINUTION OF TEMPERATURE WITH 



height of the thermometer above the mean level of the sea, ascertained by myself, 

 is 364 feet. The hours of observation were the same as above. The thermome- 

 ter has been carefully compared with a standard, and the error in different parts 

 of the scale not being uniform, it has been ascertained, and a coiTesponding cor- 

 rection applied. 



By far the greater part of the calculation of these observations, was per- 

 formed by my late friend and pupil Mr John Spens, son of Dr Thomas Spens, 

 who, had he lived, must ultimately have distinguished himself in a profession 

 which rarely fails to reward real talent. Much of the remaining calculation was 

 kindly undertaken by Air Johk T. Harrison. 



The mean temperatm-e of each month at each station at 8 a. m. and 8 p. m. 

 being taken, the mean difference for each month of the year for the whole period 

 is deduced, and hence the mean for the entire period, which gives a decrement 

 for 730 feet of ascent, amounting to 3°.27 for the morning observations, SMS for 

 the evening, or 3°.22 for both, which corresponds to 229 feet of ascent for 1° of 

 decrement of temperature. This decrement is rather rapid, and is, no doubt, 

 partly to be accounted for by the comparatively sheltered situation of the lower 

 station. 



The influence of the season of the year on the decrement of temperature is 

 particularly striking, as the following Table shews ; and that the discrepancies it 

 contains are not generally errors of observation, is jiretty clear, from the agree- 

 ment of the morning and evening columns, and various other tests, which it is 

 not necessary to mention. 



Table I. 



Calculation of the Mean Temperature of each Month during the Years 1831-32- 

 33-34-35, at Bonally and Colinton, and corrected for the en-ors of Graduation 

 of Thermometers. 



