38 EIGHTH REPORT — 1838. 



Having done so, tlie air in the bulb and tube having become rarilied 

 to the tension of that on the top of the mountain, is compressed by the 

 water which the increased pressure of tlie atmosphere as he descends 

 forces into the tube, so that when he returns to the place where the 

 barometer is at 30 inches, the height of the fluid will indicate the 

 height at which the barometer would have stood on the summit of the 

 elevation. If the barometer be not exactly at that height, a correction 

 may be applied. 



If the temperatm-e and degree of moisture of the air in the tube on 

 the mountain and at the lower station were alike, no further correction 

 would be requisite : but just as in the case of the barometer so with 

 this instrument ; for minute accuracy, a thermometer and hygrometer 

 should accompany it, and be simultaneously observed, so as to permit 

 the application of the usual corrections. 



In many cases precisely equal temperatures may be obtained at the 

 upper and lower stations by keeping the tin case supplied with water, 

 melting snow, or ice. 



In a general and i-apid survey of a country, such instruments would 

 possess value from their portability and cheapness. 



Tables intended to faciUtate the computation of HeigJtts by the Barometer. 

 By the Rev. Temple Chevallier, B.D., Professor of Mathematics 

 in the University of Durham. 



In these tables, the correction for the temperature for the air is 

 computed, so that the difference of elevation of two stations, in feet, 

 is at once found by taking the difference of two numbers correspond- 

 ing with the heights of the column of mercury at the two stations, and 

 the mean temperatui'es of the air. The table is constructed for differ- 

 ences of one tenth of an inch in the barometer, the proportionate varia- 

 tion for hundredths and thousandths of an inch being readily found 

 by an accompanying table of proportional parts. 



A table is given for the correction for the difference of temperature 

 of the mercury. 



Mr. J. S. Russell described a magnetic instrument invented by Mr. 

 Watt, of Lasswade, which, according to the experience of the inventor, 

 appeared to take positions corresponding to the direction of the wind. 



