CAPTAIN ELLIOT’S MAGNETIC SURVEY OF THE INDIAN ARCHIPELAGO. 30:i 
but the curves at the winter stations, as at Singapore during the same period, being- 
distinguished by the intensity reaching its maximum earlier by one hour than in 
spring. 
The curves in Parts 1 and 2 of Plate VIII. are both drawn to the same scale, viz. 
one scale division ( = -000240 of force) to 0-29 of an inch. The curve rising denotes 
an increase of force. The curves of the bifilar showing the changes of horizontal 
intensity, appear to partake, every one, of the same character as the horizontal 
intensity at Singapore. 
Table C . — Dry and JVet Bulb. 
The dry and wet bulb instrument was made by Cary ; it was small but accurate, 
and was in use throughout the Survey ; but the wet-bulb thermometer was broken in 
crossing from Moulmein to Madras, and at the latter station another thermometer 
was substituted in lieu of it. 
Explanation of Table C. 
The results of the dry and wet-bulb thermometers are contained in Table C, from 
page Ixxxvi to cvii inclusive. The mean variation or range of the dry thermometer 
is given in the General Table at pages Ixxxvi and Ixxxvii. At pages Ixxxviii and 
Ixxxix are given the mean variation of the wet thermometer. 
At pages xc and xci are given the mean variation of the tension and vapour, and at 
pages xciv and xcv the mean degree of humidity of the air at the different stations. 
The Tables containing only the mean results of the observations commence at 
pages xcvi and xcvii. The dry and wet-bulb thermometers at each station are given 
in succession. 
In the first line of each set is given the mean of the number of days observed, in 
the second line the diurnal variation ; then follows the mean of the w^et-bulb thermo- 
meter, the diurnal variation, and lastly, the tension or elastic force of vapour in inches 
of mercury. The formula for the tension of vapour at the dew-point is 
fU ft ^ 
J J SH’ 
where d is the difference between the dry and wet bulb, and f the tension of vapour 
at the temperature of the wet bulb. The value of f' is given in Table V. of the 
Report of the Committee of Physies and Meteorology of the Royal Society. The 
quantity of humidity in the air is found by dividing the tension of vapour at the 
temperature of the dew-point by the tension of vapour at the temperature of the air, 
and the result gives the number of proportional parts of in humidity the atmosphere ; 
and it is by this method that the Table at pages xciv and xcv is constructed. The 
Tables of the dry and wet bulb are concluded at pages cvi and cvii. 
