206 Prof. Forbes on the Diminution of Temperature 



The Bonally station is situated on the exposed northern ac- 

 clivity of the Pentland Hills, without any kind of shelter. Its 

 elevation above the mean level of the sea was very accurately 

 determined by myself trigonometrically, and the thermometer 

 hung a height of precisely 1100 feet. The exposure was the 

 north side of a cottage, which has since been allowed to fall to 

 ruin. The observations were made by Mr Johnston, the offi- 

 cer appointed by the Water Company for the inspection of 

 their works, and by his family. I have every reason to believe 

 that they were made and registered with perfect fidelity, al- 

 though, from want of practice, they may have been occasion- 

 ally erroneously entered. They were made daily at 8 A.M. 

 and 8 p.m. The thermometer was a mercurial one, now in my 

 possession, which, by comparison with a standard one, I find 

 reads pretty constantly 0°.35 too high.* The readings have 

 therefore been diminished by that quantity. 



The Colinton station was at the School-house there, and the 

 observations were carefully made and registered by my friend 

 the Rev. R. Hunter. The 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 ther- 

 niometer 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 corresponding correction applied. 

 By far the greater part of the calculation of these observa- 

 tions was performed by my late friend and pupil Mr John 

 Spens, son of Dr Thomas Spens, who, had he lived, must ul- 

 timately have distinguished himself in a profession which rarely 

 fails to reward real talent. Much of the remaining calculation 

 was kindly undertaken by Mr John T. Harrison. 



The mean temperature 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 

 736 feet of ascent, amounting to 3°.27 for the morning obser- 

 vations, 3°.18 for the evening, or 8°. 22 for both, which corres- 

 ponds to 229 feet of ascent for 1° of decrement of temperature. 



* For one year only .i spirit tliermometer was employed. 



