422 BAROMETRICAL MEASUREMENT OF HEIGHTS. 



30.053 0.017 = 30.036. This last number is to be used in the computation, 

 with the mean temperature of July at both stations. 



Towards the tropical regions, the irregular or non-periodic variations of the ba- 

 rometer, which in high and middle latitudes are so considerable as to render simul- 

 taneous observations indispensable for the measurement of heights, gradually decrease 

 and nearly cease to exist, while the monthly and daily periodic variations, which are 

 small in high latitudes, considerably increase. Within the tropics, therefore, the 

 oscillations of the barometer being far more uniform, observations made during a 

 short period of time, or even single observations, may be used for computing heights, 

 without corresponding observations, by referring them to the mean pressure at the 

 level of the sea as to a constant, provided this last has been corrected for the monthly 

 and daily periodic variation at the place. 



Table XIII. furnishes the means of applying the correction for the monthly varia- 

 tion, as described above. Table XIV., which gives the mean height of the barometer 

 at all hours of the day in various latitudes, enables the observer to correct the data 

 according to the hour at which the observations have been taken. This table is 

 from Kaemtz's Vorlesungen uber Meteorologie, French translation, p. 249. The 

 column Bossekop is from the observations of the French Scientific Expedition in the 

 North ; the column Philadelphia, from the observations at Girard College, has been 

 added. 



The correction for the hourly variation is found by taking the difference between 

 the mean of the hour of observation and the daily mean, and correcting accordingly, 

 with due regard to the signs, either the yearly mean at the sea level, or the observa- 

 tion at the upper station. 



Example. 



The barometer at Caracas, latitude 10 30' N., on the 20th of August, at 4 o'clock 

 P. M., reads 680.57 millimetres. 



In Table XII the mean height of the barometer at 

 La Guayra, lat. 10 N = 760.17 millimetres, 



By Table XIII. we find for August a correction . . = 2.95 



Mean barometer in August .....= 757.22 



In Table XIV. daily mean mean at 4 P. M. gives for 

 4 P. M. a correction . . . . . . . 1.17 



Mean barometer at La Guayra in August, at 4 P. M. = 756.05 millimetres, 



which is the number to be used for the computation of the height of Caracas. In 

 this case, however, the monthly correction, being derived from a higher latitude, may 

 be too small. Both corrections can of course be applied, with contrary signs, to the 

 observation at Caracas, leaving then the mean height at the level of the sea as a 



constant. 



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