in the cistern touches the ivory point it is at the zero of the scale and the distance to the top of the column is the observed height.
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; a flashlight will answer all purposes. There are several ways to determine tangency of the ivory point and the mercury:
Contact between the point and its shadow on the surface of the mercury.
Making a visible dent in the mercury with the ivory point; then lowering the surface until the dent disappears.
With the eye in the horizontal plane of the end of the ivory point, noting the position when the light space between the point and the mercury ceases to appear.
Observers usually prefer the last method. In practise, the first method is associated with it. The second method is fairly safe when the surface of the mercury is bright, but it is not easy to discern the dent if the mercury is tarnished. Experience will usually determine the method by which the observer will obtain the most accurate results.
Setting the vernier scale exactly to the meniscus, or rounded top of the mercury in the tube, is not always easy. The first requisite is a clean tube. The film that gathers upon the outside of the tube in damp weather catches dust and interferes with the transparency of the glass. The moral is obvious: the tube should be clean. 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.
A very great part of the value of barometer observations consists of the knowledge that may be obtained by comparisons. In order to compare observations they must be reduced to a common base; namely, a temperature of 32° F and sea level. The temperature correction, except as noted, is subtractive; the altitude correction is additive, except as the station may be below sea level. Death Valley and Imperial Valley, California, are stations in the United States to which this exception applies.
