Page:Handbook of Meteorology.djvu/221

 working condition is its sensitiveness to slight changes in elevation—for instance, the difference in elevation between the adjacent floors of a house. The response of the index should be instantaneous. An aneroid of the best type should show the difference of elevation between the top of a table and the floor.

Engineers’ Aneroid Barometers.—Within the past few years material improvements in the construction of aneroids have removed the defects noted in the preceding paragraphs. On ordinary aneroids the divisions of the pressure scale are equal, while those of the altitude circle gradually diminish. It is evident therefore that a vernier could not be used on such a scale.

In the scale graduation of the engineer’s type of aneroid, the arrangement is reversed; the pressure scale divisions diminish while those of the altitude circle are made equal. If the scale divisions represent 20 feet, the vernier subdivides them into 2-foot divisions. Many of the newer instruments have these values in scale construction; on others the scale divisions are 50 feet and 5 feet.

Although the engineer’s aneroid is compensated for temperature a slight temperature correction is advisable where the difference in altitudes is considerable. P. R. Jameson has deduced the following rule: If the sum of. the number of degrees at the two stations is greater than 100° degrees F (55° degrees C), increase the height by one one-thousandth part for each degree F in excess of 100° degrees F; if the sum of the number of degrees is less than 100° degrees F, diminish the altitude by one one-thousandth part for each degree F.

In using the engineer’s aneroid for determining altitudes the zero point may be set at the station of known altitude. For reasons explained in a preceding paragraph, such a proceeding will not do with an aneroid whose scale divisions on the altitude circle are unequal. In using such an instrument the zero point should be set at a designated position and the correction made for the variation which the reading reduced to sea level shows to be necessary.

Pocket Aneroid Barometers.—This term is applied to small instruments about 2 inches in diameter. In quality they vary from good to poor. The chief virtue about them is convenience and portability. In spite of the name, the pocket is not a