308 CAPTAIN ELLIOT’S MAGNETIC SURVEY OF THE INDIAN ARCHIPELAGO. 
through the slit, and the level of the quicksilver in the cistern is then at that point 
from which the scale is laid off ; there is therefore no correction for neutral point : 
this instrument was well made and well finished, and has yielded at all times ex- 
ceedingly satisfactory results. 
Explanation of the Table D. 
The observations made with the barometer are contained in Table D. from page 
cviii. to cxxi inclusive. 
At pages cviii and cix are given the mean variation or oscillation of the baro- 
meter ; at pages cx and cxi the mean variation of the gaseous pressure or dry column 
of air. 
From page cxiv to cxxi inclusive, are contained the mean results of the barometer 
at each station : each set contains in the first line the mean of the barometer uncor- 
rected ; then the barometer corrected to 32° ; and thirdly, the gaseous pressure which 
is deduced by subtracting from the barometric column the tension of vapour. 
Curves. 
The curves of the barometer corrected to 32°, are contained in Plate X. The 
curves are drawn to a scale of *01 of an inch of barometric pressure to 0’30 of an 
inch linear measure : the curve rises with the increase of pressure. 
Plate XI. contains the curves of the variation of gaseous pressure drawn to the 
same scale, viz. '01 of an inch of barometric pressure to 0'30 of an inch linear mea- 
sure; the curve rises with increase of pressure. 
Remarhs. 
The barometric curve has a double progression, a principal maximum and minimum 
at 9 A.M., and at 3 and 4 p.m. ; and a secondary maximum and minimum at 10 p.m. 
and 4 a.m. The intervals between the successive maxima and minima are nearly 
equal, and the degree of parallelism between all the curves is very striking; the 
range or variation appears to be nearly similar, and the hours of maxima and minima 
are identical ; one exception occurs at Madras, the maximum taking place at the 
same time, but the minimum at 5 p.m., two hours later than the minimum generally 
throughout the Archipelago. 
The variation of the gaseous pressure is given in Plate XI. ; it appears to have, 
like the barometric curve, a double progression, but the principal maximum and 
minimum occur earlier ; the a.m. minimum is more faintly expressed, with an in- 
terval of three or four hours only between the morning minimum and the morning 
maximum. The curve is likewise more irregular than the barometric, and the range 
is greater ; only a few of the curves are given, to prevent confusion from over-crowd- 
ing, and those places have been selected where observations have been taken for up- 
wards of a month. 
