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XXV. — On the Diminution of Temperature with Height in the Atrmsphere, at 

 different seasons of the year. By James D. Forbes, Esq., F.R.SS. L. Sf E., 

 Professor of Natural Philosophy in the University of Edinburgh. 



(Read 1st April 1839). 



In the year 1830, I succeeded in establishing a Register of the Thermometer 

 at the Bonally Reservoir, which formerly supplied the city water- works, being at 

 a distance of five miles in a direction south-west from Edinburgh. This station 

 is on the northern acclivity of the Pentland Hills, at a height of 1100 feet above 

 the sea. The following year I obtained coiTesponding observations at the village 

 of Colinton, situated a mile and a half north of the preceding station, and above 

 700 feet lower. Although this difference of level be not very considerable, yet, 

 as these comparative registers have been kept for nearly five years with pretty 

 unifonn results, some confidence is evidently due to the conclusions, even al- 

 though considerable difficulties opposed themselves to obtaining registers quite 

 free from exception. The interest attaching to them is the greater, that, although 

 registers have been kept at LeadhUls and other elevated stations, I do not recollect 

 any strictly comparative observations in Scotland, perhaps not even in Great 

 Britain, at two stations near one another, and diflfering considerably in level, 

 from which the important meteorological element of the decrement of tempera- 

 ture in the atmosphere could be deduced. 



The BonaUy station is situated on the exposed northern acclivity of the 

 Pentland Hills, without any kind of shelter. Its elevation above the mean level 

 of the sea was very accurately determined by myself trigonometricaUy, and the 

 thermometer hung at 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 obser- 

 vations were made by Mr Johnston, the officer appointed by the Water Company 

 for the inspection of their works, and by his family. I have every reason to be- 

 lieve that they were made and registered with perfect fidelity, although, from 

 want of practice, they may have been occasionally 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 



* For one year only a spirit thermometer was employed. 

 VOL. XIV. PART II. 4 L 



