418 



MeieoTological Register at Royacotiah, 



[April 



VII. — Meteorological Register at Royacottah. — By Lieutenant Camp- 

 BELL, Assistayit Surveyor General, 



In the annexed register, the observations at the upper station were 

 made in the hill fort, the elevation above the lower station being about 

 500 feet, as measured bj' aWollaston's thermometer and barometer, 

 and about 100 feet below the highest part of the rock on the eastern 

 side. 



The lower station is in the lower fort, distant about half a mile S. 

 W. from the upper one. 



The upper observations were registered by mj^self, but are not re- 

 gular from want of leisure to attend to them. The lower observations 

 were made by Mr. M'Mahon, head sub-assistant in the Salem and 

 Baramhal survey. 



The instruments used were carefully compared together and with 

 the best thermometers procurable, and the observations corrected ac- 

 cordingly, and the wet thermometers have been compared with the 

 Daniel's hygrometer in the manner explained in a former paper, 

 No. 17, page 295. 



The times of comparison were chosen with the intention of compar- 

 ing two barometers, but the instruments ordered for the use of the 

 survey not having been received, the intention has been frustrated. 



Want of leisure has prevented the trigonometrical measurement of 

 the situation of the two stations, but which will be completed as soon 

 as the barometers are received. 



The elevation of the upper station above the sea, by an imperfect 

 barometer, is about 3,235, the mean pressure being 26,85 inches in the 

 month of October. 



The observations of Daniel's hygrometer have been made only at 

 detached intervals, and generally only when the depression of the wet 

 thermometer was considerable, as it was intended to try the effect of 

 the elevation on the wet thermometer, and for want of a second instru- 

 ment, compared observations could not be made. 



In comparison with observations at the level of the sea, it does not 

 appear, that the depression of the wet bulb thermometer increases with 

 the altitude, although Daniel's experiments go to shew, that the qua?i- 

 tity of the evaporation varies inversely with the pressure. It will be 

 seen that the dew point is generally below the wet bulb, about 9-1 0th of 

 the difference between the wet and dry thermometer. I hope to be 

 able to have the observations at the upper station continued during 

 my absence in the district, as also those at the lower station, which I 

 hope the writer of the survey will be able to manage. 



