498 



TAIJI.K XI. Mean Hourly Velocity of the Wind for cadi Month, by Cup Anemometer ami by 

 I "i vssure-Tube Anemometer (Irrespective of Direction). 



twenty-one months covered by the pressure-tube observations is fairly accurate. The mean hourly 

 velocity so obtained is practically identical with that derived from the readings of the cup anemometer. 



Bearing in mind how essentially different in principle are the two instruments, so close an agreement 

 between them is very satisfactory, and it leaves little doubt that for the period covered by the stay of the 

 expedition at Boss Island the mean velocity of the wind there was between 10 and 11 statute miles per hour. 



This velocity is not remarkably high, and is one and a half miles per hour less than the mean hourly 

 velocity of the wind shown by twenty-five years' observations at Valencia, in the south-west of Ireland, 

 and 5 miles per hour less than the mean at the Scilly Islands; but its correctness is further confirmed by 

 the fact that the mean force of the wind, as estimated by the numbers of Beaufort's scale, for the 

 longer period of twenty-five months was 2 '5, which is equal to a velocity of about nine and a half miles 

 per hour, using for the conversion the velocity equivalents of the Beaufort scale numbers adopted by the 

 Meteorological Office. 



Speaking broadly, the variation in the mean velocity from month to month throughout the year 

 indicates pretty clearly a seasonal variation, with a maximum in winter and a minimum in summer. The 

 highest mean monthly velocity was 17 miles per hour in May, 1902, the first complete month for which 

 readings of the cup anemometer are available ; but this was considerably above the mean velocity in any 

 other month, and, disregarding small variations, there was from thence a steady decrease in strength to a 

 minimum of 6 miles per hour in January, 1903; then the force rose again to 11 miles per hour in March 

 and to 12 miles per hour in October, when it again fell away to another minimum of 7 miles per hour in 

 the following .January (fig. ''>), There appears, therefore, to be a simple oscillation of force throughout the 

 year from a maximum in winter to a minimum in summer; and, with the limited number of observations 

 available, that is perhaps all we aie justified in saying. 



