G38 Lieutenant Renny on the Constants of 



full benefit of the new formulte, I have now to make known to the Academy, 

 that, even with the help of such Table and new formula3, 1 have reason to believe 

 tliat the method of calculating heights by means of the barometer, hygrometer, 

 and thermometer, is still subject to serious error. 



A scientific periodical published at Geneva, under the direction of M. E. 

 Plantamouk, containing observations hygrometric, barometric, and thermome- 

 tric, made at the Convent of the Great Saint Bernard and Observatory of 

 Geneva, for the month of September of the year 1855, for every second hour 

 between six hours a.m., and ten hours p. m., both inclusive, fell accidentally 

 under my inspection, and I lost no time in making calculations with my own 

 formula for the height of the Convent above the Observatory for such moments 

 of the forenoon and afternoon as, according to the large Table of horary cor- 

 rections, give the horary corrections = 0. Having done this, I was sadly disap- 

 pointed at finding the calculated height to be nearly 12 metres different from 

 the true heicrht. 



I wrote immediately to M. A. Morlott, residing at the time at Lausanne, 

 requesting him to send me the observations for every month of the year 1855, 

 extracted from the same scientific periodical ; M. Morlott lost no time in 

 kindly meeting my wishes. 



With the observations for every month (as well as that of September) of 

 the year 1855, I made calculations for the height of the Convent above the 

 Observatory, at the moments when, according to large Table of horary cor- 

 rections, the corrections vanish. The mean results of such calculations give 

 very nearly the same error as the calculations for the month of September, viz., 

 twelve metres. But, though disappointed, I did not feel any, the least, distrust 

 in the new formula, being aware that the discrepancy can be explained by the 

 mistake of assuming the arithmetic mean of temperatures as the true mean, 

 which may sometimes be the case, but more often is not so. With respect to 

 such mistake, the new formula is in no way responsible for it. That which the 

 new formula proposed to do, it has done,— namely, to introduce a correct allow- 

 ance for hygrometric action, with a correct constant. Of three distinct defects 

 of the old formula two have been removed ; one still remains, and to obviate 

 its unhappy workings, we must look to Tables of horary corrections. The 

 results above alluded to are given in the following Table : — 



