22 THE METEOROLOGY OF JAMAICA 



For further particulars reference should be made to the 

 Rainfall Atlas published by the Institute of Jamaica in the 

 year 1892, which shows the average rainfall over the island, 

 for the year, and for each month of the year ; and to Weather 

 Report, No. 256 (a), which gives the average rainfall at 193 

 stations where registers have been continuously kept for at 

 least six years. 



The Winds 



Wind is caused by local differences of atmospheric 

 23ressure, which, again, are chiefly caused by local differences 

 of temperature ; and its direction at any place and time is 

 indicated by the true point of the compass from which it 

 blows. 



In Jamaica the difference between the true and magnetic 

 north is small ; at Kingston, in the year 1891, the magnetic 

 north was 2 16' to the east of the true north ; and this differ- 

 ence decreases at present at the rate of about 7' per annum. 



In order to show the prevailing direction of the wind at 

 any place for any month or year, it is convenient to construct 

 a table stating the number of times the wind was observed to 

 l)low from each point of the compass. In Kingston, for 

 instance, the prevailing direction of the sea-breeze is south- 

 east. 



Besides the direction of the wind, we further require its 

 velocity, or force. The velocity of the wind is measured in 

 miles per hour. Dr. Eobinson of Armagh invented a very 

 useful anemometer. It consists of two light rods attached 

 together crosswise, which carry four light cups at their 

 extremities. These cups and rods are free to rotate horizon- 

 tally about a light vertical axis firmly attached to the rods at 

 their point of junction ; and by means of an endless screw 

 this vertical axis can be made to register the number of 

 revolutions of the apparatus. Dr. Eobinson showed that the 

 centres of the cups rotate with one-third of the velocity of the 

 wind ; and the registering dials are made to record three 

 times the number of miles passed over by the centre of the 

 cups in consequence of their rotation, or the number of miles 

 of wind which has swept past the instrument. If, therefore, 



