414 COURSE AND LEVELS 



Barometer filled with Mercury revived from Cinnabar and well purged of air. 

 These correspondences being compared with Dalton's Table of the force 

 of steam (Thomson's Chemistry, vol. i.)give the errors oftheThermometric 

 scale, and from them the subjoined little Table is calculated by interpolat- 

 ing the intermediate numbers. Using this Table to correct the indications 

 of the Thermometer, the corresponding Barometer is taken from Dalton's 

 Table, and the height deduced therefrom in the usual manner, correcting 

 for the temperature of the air as directed by M. Ramond in calculating- 

 single observations of the Barometer. It is known that without corres- 

 ponding observations, the results of Barometrical measurement are likely 

 to be erroneous. To this error the method by boiling is also subject. But 

 in these climates, where the Barometer is so much more regular in its indi- 

 cations than in Europe, this error lies within a less compass. I find from a 

 register kept very carefully at Saharanpur that the Maximum annual range 

 is only 6 inches and in any one month not more than 4 inches. This error 

 cannot then affect the boilings by more than 300 feet in the extreme case, 

 and generally much less. But they are subject also to their own error, aris- 

 ing as well from the smallness of the scale as defect of observation. Every 

 precaution was taken to reduce this last within as narrow limits as possible. 

 Still I am afraid the error may have amounted in some cases to half a de- 

 gree. It is hardly credible that both these errors should lie the same way ; 

 and yet we see that in the Elevation of the Pass something of this kind 

 must have occurred, for the result by boiling exceeds that of Trigonometri- 

 cal measurement 551 feet. I need scarcely say that with regard to the pu- 

 rity of the water used I was most scrupulous; 1 find it difficult therefore to 

 understand the above anomaly, unless it be referable to the uncertainty of 



the correction for the temperature of the air. 



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