82 



THE HABITAT 



120. The barometer. To secure convenience and accuracy in the de- 

 termination of altitude, it is necessary to use both a mercurial and an 

 aneroid barometer. The latter is by far the most serviceable for field work, 

 but it requires frequent standardizing by means of the former. The mer- 

 curial form is much more accurate and should be read daily in the base 

 station. It is practically impossible to carry it in the field, except in the 

 so-called mountain form, which is of great service in establishing the alti- 

 tudes of a series of stations. In use the aneroid barometer may be checked 

 daily by the mercurial standard, or it may be set at the altitude of the base 

 station, thus giving a direct reading. After the normal pressure at the 

 base has once been ascertained, however, the most satisfactory method is 

 to set the aneroid each day by the standard, at the same time noting the 



pressure deviation in feet of elevation (see 

 p. 46). The absolute elevation of the var- 

 ious stations of a series may be determined 

 either by adding or subtracting this devia- 

 tion from the actual reading at the station, 

 or by noting the change from the base 

 station, and then adding or subtracting this 

 from the normal of the latter. When it is 

 impossible to check the aneroid by means 

 of a mercurial barometer, the average of a 

 series of readings made at different days at 

 one station, especially if taken during 

 settled weather, will practically eliminate 

 the daily fluctuations, and yield a result 

 essentially accurate. Even in this event, 

 the accuracy of the aneroid should be 

 checked as often as possible, since the 

 mechanism may go wrong at any time. The 

 barograph, while a valuable instrument for base stations, is not at all neces- 

 sary. These instruments can be obtained from all makers of meteorological 

 apparatus, such as H. J. Green, and J. P. Friez. Aneroid barometers reading 

 to 16,000 feet cost about $20; the price of the Richards aneroid barograph is 

 $45. Ordinary observatory barometers cost $30-$4o; the standard instru- 

 ment sells at $75-$ 1 00. The mountain barometer, which is altogether the 

 most serviceable for the ecologist, ranges from $30-$55, depending upon 

 accessories, etc. 



Aneroid barometer. 



