=120. The barometer.= To secure convenience and accuracy in the determination 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 mercurial 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 altitudes 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 various stations of a series may be determined either by adding or subtracting this deviation 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 necessary. 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–$40; the standard instrument sells at $75–$100. The mountain barometer, which is altogether the most serviceable for the ecologist, ranges from $30–$55, depending upon accessories, etc. _Slope_ [Illustration: Fig. 27. Mountain barometer: (_a_) in carrying case; (_b_) set up for use.]