CAPTAIN ELLIOT’S MAGNETIC SURVEY OF THE INDIAN ARCHIPELAGO. 307 
the carriage of the Standard. The Standard was used at the following places : — at 
the CocoSj at Batavia, at Padang, at Singapore, at Keemah, at Sarawak, and at 
Sambooanga. 
The Standard Barometer was made by Newman ; diameter of the tube 0"*532 ; the 
correction to be applied for capillary action+'003". The zero-point consists of a fine 
conical point brought to touch the surface of the mercury; this is an adjustment 
which we are instructed to make at each observation ; but throughout the Survey the 
conical point or zero-point, being once accurately adjusted, was never subsequently 
touched ; the capacity of the cistern is so much greater than that of the tube, that the 
variation of the barometer in the Tropics being not more than =j3-th of an inch, the 
level of the cistern would be but little affected thereby. The internal diameter of 
the cistern is about 4 inches ; the relative areas of the tube and cistern are as 0*25 to 
16, or as 1 to 64 ; and as the barometric column varies to the extent of ^^th of an 
inch, the cistern would be affected to of an inch, and the quicksilver there- 
fore would rise above or sink below the mean position r^roth of an inch ; a space 
which a very accurate observer might be able to detect between the conical point 
and a very bright surface, but not with the quicksilver in the barometer now in 
question, as the surface was covered with a thick film, and the glass cistern was 
somewhat dingy; for this reason, after one adjustment very carefully made at noon, 
this being about the time of the mean, the zero point of the barometer was not further 
touched during the whole series of observations ; and I never could detect at the 
hour of maximum and minimum, viz. at 9 or 10 a.m,, and at 3 or 4 p.m., any differ- 
ence in the relative position of the conical point to the surface of the mercury, and 
therefore no correction has been applied to the neutral point determined at noon. 
The Standard Barometer was by no means tight, and therefore imperfectly 
portable: when moving from one place to another, the quicksilver was constantly 
escaping by the wooden collar at its junction with the tube; this loss was supplied 
with fresh mercury, strained through leather and poured into the cistern ; and as no 
air could ever be detected in the tube, the instrument was perfectly serviceable 
throughout the whole of the Survey. The leakage at the collar was by no means 
peculiar to this barometer, as I have discovered it in others by the same maker. 
The Portable Barometer, made by Cary, was in use at a few stations during the 
Survey ; I had filled it very carefully at Singapore ; in the comparisons made with it 
and the Singapore Standard Barometer, it was generally a little lower ; but as it had 
not exactly the same range, the correction would not be constant, and therefore no 
correction has been applied. The diameter of the tube, which dimension I obtained 
from Mr. Cary, is between 0’27 and 0'28 of an inch ; the correction to be applied 
for capillary action = -}-0"’023 ; the scale is marked on brass to the twentieth of an 
inch, and can be read off by a vernier to the thousandth of an inch, being similar in 
this respect to the Standard Barometer. The gauge-point or zero-point is a slit in the 
iron cistern at its upper surface ; the quicksilver, being pressed up by means of a screw 
applied to the leathern bottom of the cistern, is raised till the light is no longer seen 
2 R 2 
