Field Practice ivith Aneroid Barometer. 357 



Using the Aneroid. 



As some instruments otherwise good vary their reading con- 

 siderably according to the position in which they are held, it is 

 well always to read with the dial horizontal. A lens may be used, 

 and in any case, to avoid parallax, try to look at right angles to 

 the dial. A mirror dial, as applied to some electrical instruments, 

 would be theoretically perfect, for when the index covered its 

 reflection, there would be no parallax. A small metal flag is 

 sometimes put at the end of the index, and this, by becoming 

 invisible, shows that the line of sight is perpendicular. The 

 ordinary index, however, if it works fairly close to the dial, and 

 is read carefully, will give very good results. If a very rapid 

 ascent or descent of several hundred feet is made, it is well to 

 give the instrument a few minutes' rest before reading it, as it 

 does not always immediately adapt itself to a large change of 

 pressure. For the same reason, testing in the vacuum chamber 

 should be performed slowly, and a pause of a minute, at least, 

 made at each point of comparison. For good work it is suggested 

 that two aneroids and a barograph, or self-recording aneroid, be 

 employed. This instrument should, if possible, be tested in the 

 vacuum chamber as are the aneroids. It is not so important, 

 however, to do this, as the range of the barograph is much smaller 

 than of the aneroids, and the risk of error consequently less. 

 Tlie barograph is left at the starting or datum point, and shows 

 every variation of atmospheric pressure there. This is better 

 than trusting to a cook or camp keeper, who may be unskilful or 

 negligent in reading the instrument left with him. The two 

 aneroids, by the substantial agreement of their corrected readings, 

 will give asssurance that both are working well, while a marked 

 difference will show that something is wrong, and lead to further 

 investigation. Each reading is booked, and the time by the 

 watch noted, so that the corresponding barograph reading may 

 be ascertained. On leaving and on returning to camp, the 

 aneroids and barograph should be compared together, and any 

 small inconsistency noted and disti-ibuted over the ol)servations. 



Thermometer readings should also be taken at the camp and 

 at the aneroid stations, but these need not be nearly so precise 

 as the aneroid readings. If expense is no object, a thermograph, 

 or self-recording thermometer, may be kept at the camp or 



