Leveling; Barometric, Trigonometric and Spirit (Paperback)

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1887. Excerpt: ... equal pressure is above C, say at E, and AE is the corresponding difference of elevation obtained by applying a statical formulae. The problem is farther complicated by the fact that the air above B also is in a state of oscillation. If the variations in pressure at the two stations were simultaneous and alike in amount, no error would be produced by the barometer gradient; but these conditions are seldom or never realized. 30. The variations in atmospheric pressure, and the consequent variations of gradient, are so complicated that it is impossible to trace the relation between cause and effect; but there are two variations that are pretty well understood. One has a daily period, and is caused by the variation in the heating effect of the sun between day and night; the second has a yearly period, and is caused by the variations of the sun's heat at different times of the year. 31. Diurnal Gradient.--It is a fact familiar to meteorologists that the pressure of the air everywhere undergoes a daily oscillation. The gradient introduced by this daily change is called diurnal gradient. The pressure has two maxima and two minima which are easily distinguishable. Near the sea-level the barometer attains its maximum about 9 or 10 A. M. In the afternoon there is a minimum about 3 to 5 p. M. It then rises until 10 to midnight, when it falls again until about 4 A. M., and again rises to attain its forenoon maximum; the day fluctuations are the larger. The daily oscillation is subject to variations in character and magnitude. The oscillation is greatest at the equator and diminishes toward the poles, but is not the same for all places of the same latitude. Within the United States it varies between 40 and 120 thousandths of an inch. Changes of altitude ...