. Report of the Boundary commission upon the survey and re-marking of the boundary between the United States and Mexico west of the Rio Grande, 1891-1896 ... one. The effect of different amounts of clouds on this unsteadiness is clearly shown by fig. 9, whichrecords a measurement of it at the same hour of different days, under almost exactly the sameconditions (soil, vegetation, temperature, etc.), but differing in the amount of clouds present when UNITED STATES AND MEXICAN liOUNDAKY. 215 the test was made. It will be seen that with the suu entirely obscured the curve is a straightline, with a
. Report of the Boundary commission upon the survey and re-marking of the boundary between the United States and Mexico west of the Rio Grande, 1891-1896 ... one. The effect of different amounts of clouds on this unsteadiness is clearly shown by fig. 9, whichrecords a measurement of it at the same hour of different days, under almost exactly the sameconditions (soil, vegetation, temperature, etc.), but differing in the amount of clouds present when UNITED STATES AND MEXICAN liOUNDAKY. 215 the test was made. It will be seen that with the suu entirely obscured the curve is a straightline, with a bright sun an abrupt curve, and with the sun partly obscured the curve takes anintermediate position. It is thought that the product of the number of vibrations per minute and their amplitudeis a better measure of the amount of unsteadiness than either factor alone. Using this productfor the values of y, and the hours of the day for values of x, the curves shown in figs. 10 and 11are obtained, which very clearly indicate the hour of maximum unsteadiness, as well as the rateof change in the amount of unsteadiness as the length of sight is increased.*. aod distance. Ttile Mountains, of vibrations by their amiilitude. In tig. 12 the product f the number and the amplitude of the vibrations is used for the valuesof y and the length of sight for the values of .r, giving a separate curve for each hour when testswere made. As in figs. 5, 0, and 7 a marked difference is seen in the curves representing thecondition of the air three hours before noon from those representing its conditions three hoursafter noon, the latter indicating much less unsteadiness; and, in general, afternoon curves areflatter than forenoon curves, as is well shown in tigs. 6 and 7. This pi-obably means that, thoughthe temperature of the air steadily increases up to 3 p. m., the temperature of the ground which,in the caiiy morning had lagged behind that of the air, now increases more rapidly than the a
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