NO. 17.] WIND. 311 



The frequency of calms has a maximum in November, and a minimum 

 in May. (PI. I. Calm days; 1 mm. = 1 day) It is above the mean from 

 August to the end of the year, and below the mean from January to July. 

 Calms are very rare as compared with cases with wind. The proportion is 

 18 to 982, or 1 to 55. 



DYNAMIC WIND-ROSES. 



Taking, for each month, the sum of the numbers expressing the velocity 

 of the wind in metres per second, for each direction of the wind (32 points), 

 and dividing the sum by the corresponding number of observations, we get 

 a series of numbers representing the mean velocity of each of the winds 

 blowing from the various points of the compass, commonly called the dyna- 

 mic wind-rose, or the wind-rose for the velocity of the different winds. 



In order to reduce the wind-directions to 16 points, I have taken for the 

 intermediate points half the sum of the velocities and half the number of 

 observations, and added these halves to the sum and to the number for the 

 adjacent 16 points; I have then added the 3 velocity-numbers and the 3 

 observation-numbers (weights), and divided the first sum by the last. ^ 



The following Table shows the numbers thus obtained. 



The exact reduction is not one half, but 0.5 sec. llVii°. °^ 0'5098. The resulting 

 error in the computed mean velocity is very small, the addition of 0'98 per cent 

 affecting the denominator as well as the numerator. 



