Field Practice tvith Aneroid Barometer. 355 



Choice of Instrument. 



Neitlier appearance nor cost is reliable evidence of quality — 

 cheap and rough-looking aneroids have sometimes performed 

 admirably ; costly and apparently highly-finished ones, owing to 

 hidden defects, the reverse. The name of a good maker is some 

 guarantee, but should not be too much relied on. Some aneroids 

 are made very small ; while admitting that diminutive instru- 

 ments ?/iaj be accurate, it is a safer course to avoid theni, and 

 prefer those with a dial not less than 2 inches diameter. In 

 thenj the mechanism is more easily made delicate enough and the 

 larger dial shows small differences of pressure more clearly. 

 Instruments with dials 4 or 5 inches diameter are better still, 

 but are costly and rather cumbersome to carry. Whether the face 

 IS open, showing the mechanism or not, is immaterial. Some 

 aneroids profess to be compensated against changes of tempera- 

 ture, others do not. In the first case, too much reliance should 

 not be placed on the accuracy of the compensation, for at best it 

 is rather rough ; in the latter the instrument should be carefully 

 tested at different temperatures, and a table of temperature 

 corrections drawn up and applied to the readings. The best way 

 is to work only when the weather is neither extremely warm nor 

 cold, and to keep the instrument in an inner pocket, when it will 

 approach to the temperature of the l)ody. 



Testing the Instrument. 



Every instrument when purchased should be carefully tested 

 in the vacuum chamber, and this test should be repeated if 

 possible every six months. Should the instrument fall or receive 

 a severe knock, it should at once be re-tested as its table of correc- 

 tions may be modified. The vacuum chamber may be exhausted 

 by a water jet aspirator, or by a mechanical air pump, and the 

 degree of exhaustion is determined by a mercurial or water baro- 

 meter, the upper end of whicli connnunicates with the vacuum 

 chamber. A water barometer 8 feet high is sufKcient for the 

 State of Victoria, where altitudes of over 6000 feet are rare. 



In testing, the aneroid should be placed in the chamber, and 

 the pressure slowly reduced by half-an-inch of mercury each time 

 till the limit is reached. Then the pressure should be slowly 

 increased by half-an-inch of mercury each time till atmospheric 

 pressure is regained. This should be done as near the sea level 



