370 THE POPULAR SCIENCE MONTHLY. 



cury, and to air in the tube. These errors may be corrected by 

 comparison with a standard instrument. 



The form of barometer known as the aneroid is also frequently 

 used for the determination of heights, a graduated scale being 

 added for this purpose. This scale is graduated by means of one 

 of the barometric formulae already referred to. The aneroid 

 barometer usually consists of a metallic box from which the air 

 has been exhausted, and differences of atmospheric pressure are 

 recorded by a system of levers which act on an index hand which 

 marks the reading on a graduated scale. In some forms of ane- 

 roid the box is not completely exhausted of air, and these are 

 called " compensated aneroids," but the name is misleading, some 

 of these instruments being more sensitive to changes of tempera- 

 ture than those not compensated. The aneroid is a very handy 

 instrument and easily used, but for the purpose of measuring 

 heights it is much inferior to the mercurial barometer. In some 

 instruments the altitude scale is fixed at a certain reading, say 

 thirty or thirty-one inches, and in others it is movable, and can 

 be adjusted to any reading required. The latter seems the most 

 convenient plan. In either case it is clear that absolute elevations 

 above the sea level can not be determined with this instrument 

 with any approach to accuracy, as there is no way of making the 

 necessary corrections for variations in pressure, temperature, etc. 

 The aneroid barometer should, therefore, be used only for finding 

 differences of elevation, and for this purpose it will give fairly 

 good approximate results in cases where extreme accuracy is not 

 required. 



To show the degree of accuracy attainable by the barometric 

 method, two examples may be cited. From readings of a mercu- 

 rial barometer at the summit of Mont Blanc and at the Geneva 

 Observatory made by MM. Bravais and Martins in the year 1844, 

 the height of the mountain above the level of the sea was com- 

 puted to be 4,815"9 metres, or 15,800'44 feet. Corabeuf found by 

 trigonometrical measurement a height of 15,783 feet, or 17'44 feet 

 less than that indicated by the barometer. 



The height of Mount Washington, in the United States, was 

 found by a spirit level to be 6,293 feet above sea level, while the 

 barometric method gave 6,2917 feet, a close approximation. In 

 some other cases, however, much larger differences have been 

 found, and the good agreements quoted above may perhaps be 

 considered as accidental. Gentleman's Magazine. 



The proper method of using a tent iu Albania, according to Mr. W. B. Cozens 

 Hardy, " is to pitch it, and then sleep under a tree three hundred yards away. 

 The tent, and not its owner, is bullet-riddled in the morning." 



