328 The adaptation of the Aneroid for surveying in India. [No. 4. 



exactly at the survey station, the cisterns were six inches above ground. 

 The following is the result. 



No. 2 No. 3 No. 5 



Barometers at Balcairn at 3 p. m 29.882 29.849 29.874 



Barometers at Survey Station at 6 p. m.. . 28.966 28.986 28.984 



Difference .916 .863 .890 

 Mean .889. The temperature at Balcairn was 5° higher than that 

 above : no correction for this was at this stage made. 



Aneroids as above, 29.945 29.860 29.850 



28.900 28.888 28.950 



Difference 1.045 .972 .900 

 Mean .972. Difference from barometric mean .083 

 The following experiments were made at the level of the sea at half 

 tide, and at Balcairn, on the summit of the rock close by ; 



No. 2 No. 3 No. 5 



Barometer, lower 29.936 29.914 29.926 



Barometer, upper .860 .836 .856 



Difference .076 .078 .070 



Aneroid, lower 29.910 29.830 20.923 



Aneroid, upper 840 .770 .850 



Difference .070 .060 .073 



The mean depression of the aneroids was thus .067, that of the 

 barometers was .074 — difference .007 — seven thousandth parts of an 

 inch. When the difference of level between two places is trifling, one 

 tenth of an inch of depression represents 100 feet, so that Balcairn by 

 this is about seventy feet above the level of the sea. Leisure was not 

 allowed to make any of these observations with the care required : the 

 barometer when carried about in the sun ought always to be allowed 

 to hang in the shade for a sufficient length of time to permit the mer- 

 cury in the cistern and tube to obtain the same temperature as that in 

 the attached thermometer, forbidden by Sir J. Herschell to be immersed 



