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 exhibits 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 the 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 — h = A + BT + C (t) -f D); 
in which a, b, 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, coefficients 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 + 0-107 T, . (1) 
and a' — b'= 35-14-f-0-107 T" —0-004 (402 — a).(2) 
These formulas 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 Golden and 
Mount Marcy, where the conditions expressed in (2) w-ere not satisfactorily answered. 
As is not uncommon, even with good instruments, the column of No. 275 exceeded that of 
No. 366 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 part at least of this difference of columns might arise from a small portion of air in the 
summit of the tube of No. 366; and that consequently the correction above, instead of being 
constant, would depend upon the temperature and volume of the included air. On this hypo¬ 
thesis, which, however, I 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 
