=50. Watch Aneroid.=--Subsequently the aneroid has been further reduced in size and it can now be had from an inch and a quarter to six inches in diameter. The smallest size can be enclosed in watch cases, fig. 35, or otherwise, so as to be adapted to the pocket. By a beautifully simple contrivance, a milled rim is adjusted to move round with hand pressure, and carry a fine index or pointer, outside and around the scale engraved on the dial, or face, for the purpose of marking the reading, so that the subsequent increase or decrease of pressure may be readily seen. These very small instruments are found to act quite as correctly as the largest, and are much more serviceable. Besides serving the purpose of a weather-glass in the house or away from home, if carried in the pocket, they are admirably suited to the exigencies of tourists and travellers. They may be had with scale sufficient to measure heights not exceeding 8,000 feet; with a scale of elevation in feet, as well as of pressure in inches, engraved on the dial. The scale of elevation, which is for the temperature of 50 deg., was computed by Professor Airy, the Astronomer Royal, who kindly presented it to Messrs. Negretti and Zambra, at the same time suggesting its application. Moderate-sized aneroids, fitted in leathern sling cases, are also good travelling instruments, and will be found serviceable to pilots, fishermen, and for use in coasting and small vessels, where a mercurial barometer cannot be employed, because requiring too much space. [Illustration: Fig. 35.] Admiral FitzRoy, in a communication to the _Mercantile Marine Magazine_, December, 1860, says:--"Aneroids are now made more portable, so that a pilot or chief boatman may carry one in his pocket, as a railway guard carries his timekeeper; and, thus provided, pilots cruising for expected ships would be able to caution strangers arriving, if bad weather were impending, or give warning to coasters or fishing boats. Harbours of Refuge, however excellent and important, are not always accessible, even when most wanted, as in snow, rain, or darkness, when neither land, nor buoy, nor even a lighthouse-light can be seen." =51. Measurement of Heights by the Aneroid.=--For measuring heights not exceeding many hundred feet above the sea-level by means of the aneroid, the following simple method will suffice:-- Divide the difference between the aneroid readings at the lower and upper stations by .0011; the quotient will give the approximate height in feet. Thus, supposing the aneroid to read at the Lower Station 30.385 inches. Upper Station 30.025 ------ Difference .360 ====== Divided gives .360/.0011 = 327 feet. As an illustration of the mode in which the aneroid should be used in measuring heights, the following example is given:--