Jacques Wardlaw Redway

“ Because the cistern is partly filled only, a marine barometer is easily put out of adjustment during transportation.
When a barometer is inclined so that the mercury is near the top of the tube, a slight lengthwise movement will cause it to flow to the top, striking it with an audible “click.” There is a tradition that the character of the vacuum can be determined by the character of the sound; but inasmuch as trained experts are sometimes deceived, the value of the click as a test is uncertain ”

“ If a mercurial barometer is used at the location whose altitude is to be determined, it should be allowed to “rest”—that is, to adjust itself to the altitude—for a few days, if possible.
The readings may be made hourly at the same time at both stations, the height of the mercury, time and temperature correction being noted. This may be repeated for several days until the reduced readings are constant. If the stations are not far apart, a single series of observations may suffice; if they are more than 25 miles apart differences in pressure other than those due to altitude may interfere. ”

“ When the temperature is materially below zero, F, at an altitude of 5000 feet, more or less, the readings are often of uncertain value. The moral is obvious. Uniform and constant conditions are necessary for uniform results.
The compensation for capillarity is usually corrected by adjustment of the scale. Mercury does not “wet” glass; therefore the surface of the tube not only tends to retard the rise of the column, but prevents the mercury from assuming a level surface at the top. The rounded surface is the meniscus, the shape of which changes from time to time, as pressure varies. ”

“ The term “sea level” is differently interpreted in different localities, “Mean tide,” “mean low tide,” “mean high tide,” and “mean sea level” are used. If the local usage does not conform to that of established Federal usage, the nearest established Plane of Reference [3] practicable should be sought as a starting point. The Weather Bureau has established a specific elevation for each of its stations; the nearest station practicable therefore may be taken as an initial point. ”

“ A clean room, free from dust, is desirable for barometers. Dust is not preventable, but it should not be permitted to accumulate on instruments. A soft, damp—not wet—cloth will remove and gather it without scattering ”

“ The refraction of the glass has a tendency to produce a “drop,” making it slightly difficult to adjust the two edges of the vernier shutter so that the line of sight is precisely tangent to the meniscus. The eye, of course, must be in a line with the edges of the windows. ”

“ Adjusting the surface of the mercury to the ivory point is best accomplished in many instances by the use of artificial light. Where convenient an extension socket to the nearest light plug is the best method ”

“ When a barometer is sent by messenger it should be sent either in a leather case or a box designed for the purpose, with the handle so placed that it must be carried cistern uppermost. It should not be allowed to rest with the end on the floor of a moving vehicle. ”