reading off at the upper station only. This cannot give true results, as the scale can only be correct when the zero is in the position in which it was graduated. Aneroids, with fixed scales, are correct only at one given temperature, and at one only. It is true that, by using the aneroid as an ordinary barometer, and reading off in inches, we can, by a long calculation, arrive at a correct result, but when some hundreds or thousands of observations have to he made, the process of reduction will evidently be a wearisome one; and if we can read off correctly in feet, at once, it will be a great advantage.
The accompanying woodcut shows Field's aneroid as an instrument 2¾in. in diameter, and a little more than an inch in thickness, thus fitting easily in the waistcoat pocket. Its novelty consists in the fact that the _(page_28_illustration).jpg)
lid marked A A is moveable, and bears nicks, which lock with a pin on the fixed lower case, These nicks are marked from 30° F to 75° F., and the rim must be moved round until the nick bearing the temperature of the air at the time of observation is fixed by the lock pin. The scale of feet on the inside of the rim is then correct for that temperature. The altitudes are in all cases determined by taking two readings, one at the lower station and another at the upper, and then subtracting.
The principle, in fact, is this: an ordinary aneroid scale is correct at one temperature only, and is incorrect at every other: but "the very fact of the scale becoming inaccurate for the temperature for which it was graduated renders it practically accurate for some other temperature, so that the shifting of the scale into certain fined positions answers the same purpose as if the original scale were altered to suit various temperatures of the air."
Another improvement which has been introduced has been the jewelling of the working parts of the interior like a watch, the effect of which has been to increase its sensitiveness in a marked degree, and to render unnecessary the repeated tappings by which an ordinary aneroid must be subjected. It bas further been tested and verified at Kew Observatory. A certificate from Kew should be demanded by every purchaser of meteorological instruments, of whatever description. The aneroid thus constructed is not intended to measure very great elevations; indeed, its scale does not extend above 5,000ft., but this permits of reading with accuracy, by means of a pocket magnifier, to 2ft. or 3ft. To set it, we obtain the temperature of the air, by means of a small whirling thermometer, (thermometre fronde,) which is strung round the hand by a cord, thus giving true shade temperature even under the direct rays of the sun; but this is really hardly necessary, as if it be set approximately within 5° F. a nearly correct result will be obtained.
The following instances may be taken as a fair test of the accuracy and simplicity of the aneroid:—
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