lo THE METEOROLOGY OF JAMAICA 



was subtracted from all barometric readings reduced to the 

 sea-level. 



This reduction to standard gravity should be incorporated 

 with the reduction to the Kew standard ; and for any altitude 

 not exceeding a thousand feet or so, a small table should be 

 drawn up further reducing to 32 and to sea-level for each 

 degree of temperature as usually experienced at the station 

 in question, always supposing that the barometer is kept 

 where its temperature as shown by the attached ther- 

 mometer will not greatly differ from the temperature of 

 the air as shown by the dry-bulb thermometer in Stevenson's 

 screen on the lawn. 



Commencing observations, the first thing which attracts 

 our attention is the diurnal variation of the atmospheric 

 pressure ; two small waves pass daily with great regularity, 

 the crests at 9.30 a.m. and 10.30 p.m., and the hollows at 

 4 p.m. and 3.30 a.m. The wave which passes during the 

 daytime is about twice as great as the one which passes 

 during the night ; and neither of them is affected by heavy 

 local rains, nor yet by cyclones, however widely extended. 



Table III. gives the correction for this diurnal variation 

 to be applied to the reduced reading of the barometer ; and 

 it cannot be neglected when accuracy is required. Thus, 

 suppose about the middle of August that the reduced reading 

 at 7 a.m. was 29*910, and at 3 p.m. 29*869 : by neglecting 

 the diurnal variation it would appear that the pressure was 

 falling ; but by applying the correction it will be found that 

 the pressure was really steady, the corrected reading being 

 29*900 both at 7 a.m. and 3 p.m. 



Now, the mean pressure of a day at any place is the sum 

 of the twenty-four reduced readings of the barometer taken 

 at each hour of the day and night divided by 24 ; the 

 mean pressure of a month is the sum of the daily means 

 divided by the number of days in the month ; the mean 

 pressure of a year is the sum of the monthly means divided 

 by 12 ; and the mean pressure at the place is the sum 

 of the annual means divided by the number of years of 

 observation. 



Table IV. gives the mean pressure at Kingston sea-level 

 for every ten days throughout the year ; and by its use we 



