ARISING FROM DIFFERENCE OF ELEVATION. 



13 



been lately taken with great care at the Grey Lock Observatory on Saddle Mountain in 

 Massacluisetts, and at Williams Collogo. The distance in a direct line between the two 

 stations is about 5^ miles, and I ho diflerence in level 2767 feet; the lower station being 

 about 800 feet above the level of the sea. The observations were taken at intervals of 

 two hours from five o'clock in the morning till nine in the evening, and continued on 

 three successive days. The mean result shows a diminution in temperature of 1° for every 

 337 feet of ascent. 



The above are all the direct observations that I have been able to obtain to determine 

 the law in question ;* and arranged in tabular form, they stand as follows : 



The rule commonly laid down, is to allow 1° for every 300 feet of ascent ; but from the 

 above table, that would seem to be too much, and that 1° to 325 feet would be nearer the 

 truth. It ouo^ht, however, to be noticed that all the observations I have quoted were 

 taken, so far as I can learn, in the wanner part of the year. Observations taken in 

 winter might modify the result. 



It has been thought that where the slope of the country is gradual, the diminution of 

 temperature from elevation is less than when you compare it on isolated peaks of precipi- 

 tous mountains with the plains belov^, or when you ascend in a balloon. IVIr. Kirwan 

 estimated that when the rise was not more than six or seven feet per mile, the decrease was 

 not more than 1° in 800 feet ; and for any ordinary rise, not more than 1° in 400 feet. 

 By the experiments of Dr. Hutton, near Edinburgh in Scotland, it was 1° in 270 feet; but 

 I do not think his data so satisfactory as those which are furnished by the observations 

 taken at the academies in your State. The latter have been taken for a considerable 

 number of years, according to fixed and uniform rules, and at the same time of day ; and 

 though it is possible there may be cases where the observers may not have been as careful 



•Since writing tlie above, I have noticed a series of observations taken at Ithaca (New- York), by Messrs. Cogswell 

 and Eddy, in the year 1S37. The distance between the stations was about half a mile, and tlie ditference of level 300 

 feet. The observations were continued daily through the year, and the result is very remarkable, showing a difference 

 in the mean temperature of 3". 99, which is about 1° to every 75 feet of ascent. There appears to be some mistake 

 in the record, typographical or otherwise, a.s the annual result does not agree with the average of the semi-monthly 

 results. If the localities are favorable, I hope the experiment will be repeated. 



