52 



showing small differences of elevation, but at the best all direct indi- 

 cations of elevation from readings on the dial of the aneroid are only 

 roughly approximate. 



106. Adjustable scale of elevations^ how used. — If the scale of feet 

 is adjustable on an aneroid for moderate changes of elevation, it may 

 be conveniently used as follows : If a tourist is about to set out on a 

 short expedition to an elevated point, starting at a station of which 

 the elevation is known, let him set the movable scale of the aneroid 

 so that the proper graduation, marking the known elevation, stands 

 opposite the index hand. On reaching the elevated point the position 

 .of the hand on the scale of feet will now indicate approximately the 

 new elevation provided, of course, the scale has not been shifted, and 

 provided further that the real air pressure was uniform throughout 

 the vicinity and did not change at all during the time occupied in the 

 expedition. If this time was several hours or the distance consider- 

 able, the result may be very greatly in error. Suppose we know our 

 elevation to be 500 feet and we set the scale to this point. To-morrow 

 the aneroid may indicate only 200 or even 800 feet, the new value 

 being a direct result simply of the changes in air pressure. 



A variation of one-tenth of an inch in the barometric pressure 

 affects an observation of altitude by from nearly 100 to nearly 150 

 feet, according (1) to the elevation itself and (2) the temperature at 

 the time. On this account and because of the large accidental errors 

 to which aneroids are subject, as described above, they are unreliable 

 in the determination of elevations. 



lOY. To determine heights as well as can be done by pressure meas- 

 urements, it is necessary that simultaneous observations, not only of 

 the barometer but of the temperature and humidity of the air, be made 

 at one or more adjacent stations of which the elevations are known. 

 These observations, by means of suitable tables, will give the differ- 

 ence in elevation of the stations, and the mean result from a large 

 number of such simultaneous readings will give a fairly accurate 

 value for the desired elevation, especially if the differences of eleva- 

 tion and the distances between the stations are not very great. 



108. The care and preservation of harovieters. — A barometer is a 

 very delicate instrument, and in general must be handled with great 

 care ; therefore observers in handling a barometer should first inform 

 themselves as to the best methods to follow and the various precau- 

 tions to observe, as embodied in the instructions given below. 



109. When a new barometer is received, in unpacking it should be 

 lifted cistern uppermost from the box and all wrappings removed 

 after placing the barometer in a horizontal position. Wlien moved 

 about, the cistern end should be carried uppermost. 



110. To turn the barometer tube end up, bring it first gradually to 

 a. horizontal position, watching for a small bubble at the cistern. 

 This should never be very large, nor should it be absent, in which case 

 there may be serious pressure from within, tending to force the mer- 

 cury out through joints of the cistern, etc. If necessary, the adjust- 

 ing screw should be turned so that the bubble is not larger than a 

 space within which a 10-cent coin could be placed. The tube may 

 then be gradually elevated to an upright position. The mercurial 

 column should not be lowered until the instrument is safely sus- 

 pended from a hook. 



