OF METEOROLOGICAL INSTRUMENTS. 



33 



FIG. 119. 



PIG. 100. 



THE ANEROID BAROMETER. 



This ingenious and elegant instrument is now regarded as almost indispensable to 

 all who take interest in the weather, whilst, to travellers in particular, it presents 

 advantages which hitherto they could not obtain. Before the introduction of the 

 aneroid, limited indeed were the means of those, who, moving from place to place, 

 desired in their progress to take reliable notice of meteorological phenomena, whilst the 

 measurement of heights by any convenient or simple and portable arrangement was 

 quite out of the question. Not only are all these difficulties entirely overcome by this 

 instrument, but the older fragile form of barometer used at sea is almost entirely 

 superseded. The action is obtained by the compression by the atmosphere of a thin, 

 flat, circular metallic box, which is deeply corrugated to increase its elasticity, and from 

 the interior of which the air has been carefully removed ; the upper and lower surfaces 

 are held in a state of tension or separation from each other by means of strong springs ; 

 the atmosphere pressing with varying force on these surfaces, conveys action to smaller 

 springs, and thus show by hands on the dial the variation of heights, as well as changes 

 of the weather. 



No. 100 shows the general interior arrangement. In the measurement of small 

 differences of height, as well as great elevations, the improved aneroid is alike 

 interesting and valuable; not only does it show with precision the differences in 

 height between one room and another, or the varying gradients in travelling on a 

 railway, but it is now so improved as to show with much precision elevations up 

 to fifteen to sixteen thousand feet. 



The sizes vary from those of a small watch, to those of the largest dials ; the form 

 being usually circular, though that is also varied according to the position or place, such 

 as large halls, public buildings, etc., for which it may be required. 



