BAROMETRICAL MEASUREMENT OF HEIGHTS. 



425 



Table XIV. shows that, after all irregular variations of the barometer have been 

 eliminated, there remains a double period of rise and fall within the twenty-four 

 hours, and that the amplitude of these daily oscillations is greatest within the tropics, 

 and goes on diminishing towards the polar regions. 



According to Kaemtz, the mean time of the daily maxima and minima, or the 

 mean tropic hours for the northern hemisphere, are as follows : — 



h. 



The minimum of the afternoon is reached at 4.05 P. M. 



The maximum of the evening is reached at 10.11 P. M. 



The minimum of the night is reached at 3.45 A. M. 



The maximum of the morning is reached at 9.37 A. M. 



Even in temperate and high latitudes these diurnal variations, though small, must be 

 taken into account, if great accuracy is required, in reducing corresponding obser- 

 vations made at a somewhat different hour to the time of the observation at the 

 station the height of which is to be determined. But in so doing, it must be remem- 

 bered that the times of the minima and maxima change with the seasons, as is shown 

 by Table XV. from Kaemtz, p. 251 of the French translation. 



XV. TROPIC HOURS OF THE DAILY VARIATION OF THE BAROMETER AT HALLE. 



LAT. 51° 30' N. 



This shifting of the times of maxima and minima with the seasons diminishes with 

 the latitude, and tends to disappear towards the equator, with the inequality of the 

 davs and nights. The elevation above the level of the sea also causes a change in 

 the tropic hours of the daily variation which is not yet sufficiently studied. 



Table XIV. gives evidence that the amplitude of the hourly oscillation is greatest 

 under the equator, and gradually decreases towards the pole. Kaemtz computes its 

 mean value in various latitudes and at the level of the sea, as follows : — 



XV'. AMPLITUDE OF DAILY VARIATIONS IN VARIOUS LATITUDES. 



The amplitude also decreases with the elevation, at least in our latitudes ; it was 

 found to be on the Faulhorn, in Switzerland, 9000 feet above the sea level, 

 0.27 millimetres, while it was 0.90 millimetres at Geneva. 



D 85 



