H. A. Hazen — Air-pressure at High Stations. 109 



lie on opposite sides of Mount Washington and at distances 

 altogether too great to give the best results; if, however, we 

 take the mean of the two we shall obtain an approximate value 

 for the base of the mountain. 



It is evident from Table II that during the prevalence of 

 relatively high pressure, elevations, computed barometrically, 

 will in general be too small, and, on the other hand, when the 

 pressure is low the computed heights will be too great. This 

 tact also explains the coincidence of too great computed heights 

 with high winds, for the reason that the highest winds always 

 occur with relatively low pressure ; on the contrary, when the 

 wind is light the pressure is generally high. We may conclude, 

 then, that the same cause which produces the maxima and 

 minima of pressure, controls also the different values of com- 

 puted elevations, and that high winds are rather an accompani- 

 ment than a direct cause of the same variations. 



May not the apparent lagging of the axes of "highs" and 

 "lows" be due to the effect of varying temperature. The gen- 

 eral tendency of high temperature being to expand the air and 

 force it from the lower levels, above the summits of mountains, 

 and of low temperature to produce an opposite effect, we should 

 expect at elevated stations high pressure with high temperature 

 and low pressure with low temperature. This principle is well 

 illustrated by the Signal Service observations of pressures on 

 Mount Washington and Pike's Peak. In January the mean 

 pressures are '2.S"--' : J98 and 17"'512 at the two points respectively, 

 while in July they are 23"'894 and 18"'078. The same results 

 would follow the accidental fluctuations, whenever there might 



be a steady, gradual rise or fall in temperature for a sufficient 

 period of time. When a "low" has passed a station at sea- 

 level, the temperature frequently falls steadily with a west 

 wind and the result is a contraction of the air, which causes its 

 withdrawal from the upper atmosphere and a further fall in 

 pressure there. This process will continue until the fall caused 

 by the low temperature is counterbalanced by the rise due to 

 the advancing high. The reverse of this may take place at the 

 passing of a "high." In order to ascertain the influence of 

 varying temperature as suggested above, I have proji 

 pressure curves for a singular case at Pike's Peak, in which the 

 minimum occurred forty hours earlier at Denver than at Pike's 

 Peak; also, for a case in which the minimum for Burlington 

 and Portland occurred twenty-four hours earlier than on Mount 

 Washington. 



Referring to the curves and the temperatures at Pike's Peak, 

 we see that on November fourteenth, at the 7 A. M. observation, 

 the mean, temperature of the air column was comparatively low 

 and the pressure at a maximum at Denver; the temperature 



