196 GEOLOGY OF THE SECOND DISTRICT. 



made by my brother, whose carefulness and skill in observing had been tested. The eleva- 

 tion of this station above tide was 374 feet, determined by the spirit level as above. 



The table below e.xliibits the notes as they were taken from the instruments, with their 

 respective calculated elevations. To convey a just idea of the agreement or discrepancy of 

 the results, I have presented all that were made, with the exception, I think, of five or six, 

 some of which bore evident marks of faultiness in their observations. The two numbers in 

 the fifth column, corresponding to each date, are the upper and lower readings of tlie baro- 

 meter, which are recorded instead of their sum, as furnishing a means of verifying the 

 accuracy of the observations, particularly in reference to the temperature of the mercury, 

 which is liable, without extreme care, to a false indication by the attached thermometer, of a 

 number of degrees. I have ascertained that the condition to be satisfied in order to be assured 

 of accuracy in this respect, for any syphon barometer, is contained in the equation 



a — & = A + BT + C (w + D) ; 



in which a, h, are respectively the upper and lower readings ; T, the temperature of the 

 mercury as indicated by the attached thermometer ; v, the distance of the superior mercurial 

 surface ; and A, B, C, D, coelScients which differ in different barometers, but are constant 

 in the same. The appropriate conditions for the barometers No. 275 and No. 366, the former 

 of which was used at Burlington, I have found to be, respectively, 



a —b = — 2-17 + 0-107T, (1) 



and a' — h'— 35-14 + 0- 107 T'— 0-004(402 — a) (2) 



These formulae have been employed in rejecting some of the faulty observations referred to 

 above, and, assuming the correctness of T, in correcting the elevations of Lake Colden and 

 Mount Marcy, where the conditions expressed in (2) were not satisfactorily answered. 



As is not uncommon, even with good instruments, the column of No. 275 exceeded that of 

 No. 360 by 2 '50 millimetres, which I consequently added to the sum of the upper and lower 

 readings of the superior barometer. This difference between the columns is a mean derived 

 from a comparison of more than one hundred sets of observations, in which care was taken 

 to secure as great a degree of uniformity in the temperatures of the atmosphere and mercury 

 as possible, and to exclude all causes of change in the columns which were not equally 

 operative in each, except those depending upon peculiarities in the constructions of the 

 instruments themselves. 



Various experiments, which it is needless at present to detail, suggested the possibility 

 that a iJart at least of this difference of columns might arise from a small portion of air in the 

 summit of the tube of No. 306 ; and that consequently tlie correction above, instead of being 

 constant, would depend ujion the temperature and volume of the included air. On this hypo- 

 thesis, which, however, 1 was prevented from verifying to the extent desired, by the loss of 

 one of the barometers, I made the correction (451 — a) x 0'021 x (16 — T), in metres, 

 which is additive or subtractive according as T is less or greater than 16. But to whatever 



