THE BAROMETER 9 



This can be done with considerable accuracy ; and con- 

 Tersely, when a barometer is read at a given elevation, the 

 reading can be reduced to the level of the sea. 



Table 11. gives the reduction of the barometer to the sea- 

 level for different temperatures of the air at the sea-level. 

 When the temperature of the air at any given elevation is 

 known, the temperature of the air at the sea-level can be 

 obtained approximately from Table YI. ; thus, at the Blue 

 Mountain Peak, which is 7423 feet above the level of the 

 sea, the temperature falls on an average 23"5 below the 

 temperature at the level of the sea ; and at this elevation 

 it makes little difference in the fall of temperature whether 

 we consider the fall at the hottest or the coolest time of the 

 twenty-four hours ; but at other elevations the difference may 

 be sensible, and should be allowed for if necessary. 



The next correction to be applied to the barometer is 

 for all the instrumental errors combined, including any very 

 slight difference in the specific gravity of the mercury em- 

 ployed, and that of the mercury in the standard of the Koyal 

 Society. All barometers intended for accurate purposes are 

 therefore sent to the Kew Observatory to be compared with 

 the standard ; the differences are carefully noted, and may 

 be termed the reductions to the Kew standard. This reduc- 

 tion for a well-made instrument will never exceed a few 

 thousandths of an inch. 



Finally, a small correction depending on the latitude 

 should be applied in consequence of the variation of gravity 

 with the latitude. Terrestrial gravity is greatest at the 

 X)oles and least at the equator ; so that a column of mercury 

 30 inches high at the equator would balance a column of 

 mercury of only 29*84 inches at the poles, the two columns 

 having the same temperature. Consequently, in comparing 

 barometric pressures in different latitudes, it becomes necessary 

 to adopt some standard gravity; and it has been agreed to 

 adopt gravity at the sea-level in lat. 45" as such standard. 



Different nations have recently adopted this correction at 

 different times ; and I believe that it was agreed that all 

 nations who had not previously done so should adopt it 

 on January 1, 1901. In Jamaica the correction was adopted 

 January 1, 1896; and 0*063 inch, corresponding to lat. 18^ 



A 2 



