1848.] Observations made on a Botanical Excursion. 359 



country ; but amongst the hills, the changes in the state of the atmo- 

 sphere are so sudden and their effects so local, that the Barometer there 

 often continues rising during 12 hours or more when the mercurial 

 column is stationary or even falling at Calcutta, and vice versa. There 

 are even instances on record of moderate elevations determined from 

 monthly means, varying upwards of one hundred feet ; that of Gurgaon 

 is from the mean of one month's observations, 868 feet ; by another 

 month's 817. Nasirabad* (by Lt. Col. T. Oliver) from one month's, 

 1430 feet, from another 1539 feet: the mean of two following years' 

 observations again shew a perfect accordance. In cases where there 

 have been continued steady weather and coincidence in the fluctuations 

 of the column, much reliance may be placed on the height so computed 

 from a comparison of the indications of good Instruments, provided the 

 proper correctionsf be employed. A little practice will give the obser- 

 ver some idea of what indications are most trustworthy. When the 

 elevation is to be calculated from the means of several maximum or mini- 

 mum observations, it is necessary to take into account the daily range 

 at the two stations ; which varies not only at differe nt positions, bu 

 with each month ; for instance in February of one year at Calcutta 

 the mean daily tide is 0.147. ; and at Kotgurh as low as 0.028. 



A considerable amount of difference in elevation is also due to the 

 formula employed ; that which I have adopted is the usual one modified 

 by Daniel, who corrects the specific gravity of the atmosphere by the 

 Dew-point. J In India the humidity of the air varies so greatly in differ- 

 ent stations, that I think this correction should not be overlooked. It 

 is to be remarked however, that (as Mr. Muller first pointed out to me,) 

 in the last edition of Daniell's work, there is a discrepancy in my results 

 as worked by the rule or by the example : the method adopted as shewn 

 by the example, seemed to us the most correct, and except when other- 

 wise stated this is always employed. 



A very excellent formula is that used at the Surveyor General's office, 

 for a copy of which I am indebted to Captain Thuillier, an officer to 



* Jour. As. Soc. 1835 (January, No. 37. p. 49. 



-\- In those Barometers of Troughton and Simms, used in India, I do not find a 

 measure of the diameter of the tube to accompany the Instrument, and the correc- 

 tion for capillarity is hence too frequently disregarded. The diameter of the bore 

 is generally 0.25 inch, and the consequent correction 0.040 always to be added. 



I Daniell's Meteorological Essays, Ed. 2. (1845.) v. 2, p. 46. 



