AND NEIGHBOURING SEAS DUKIKG 1897. 213 



case a wind of force 3*9 was not shown to be capable of producing 

 a current of velocity 15 miles per diem from rest; it is merely the 

 force of wind sufficient to maintain such a current when already in 

 motion with that velocity. It provides a datum for estimating the 

 mean velocity of a permanent drift when the velocities of the \vinds in 

 the same region are known, or conversely. But it does not establish a 

 means of determining the effect of the wind upon the sea in a region not 

 subject to a regular circulation, such as the English Channel, if I am 

 right in my attitude upon this point. The currents in the Channel have 

 to be raised practically from rest, since the great variability of the 

 winds prevents the formation of currents with any such momentum as 

 that of the permanent currents. 



As, however, the wind frequently blows from the same quarter 

 for several days in succession, its effect upon the water will be relatively 

 greater under these circumstances than when blowing intermittently. 

 The momentum of the drift produced by high winds will be con- 

 siderably greater than for low winds, and the waves raised by strong 

 winds will enable these winds to exert a propelling force upon the 

 water, in addition to the normal dragging force. All these circum- 

 stances seem to me to show that the velocity of a current raised in 

 a given time will be relatively very much greater when raised by a high 

 wind than by a low wind. Without therefore attempting to treat 

 the matter from the difficult point of view of hydrodynamics, it never- 

 theless seems justifiable to regard the velocity of the current as 

 approximately proportional to the pressure rather than to the velocity 

 of the wind, the pressure being a constant fraction of the square of the 

 velocity. 



§ 5. Empirical Datum. — On examining the results of the year's 

 experiments one case of drift stood out clearly from the remainder 

 as supplying a datum, comparatively free from sources of error, in 

 regard to the measurable efl'ect of the wind in producing a surface 

 drift, viz., batch no. XXXIX. Out of five bottles put out near the Eddy- 

 stone on April 22ud one was recovered exactly three days later (73 

 hours) at Portscatho, situated 25 miles to the westward. The wind 

 during this period was remarkably uniform in force and direction from 

 the eastward, owing to an area of high pressure lying to the N.E. of our 

 islands; and the duration of drift was sufficiently long to allow the 

 oscillating effect of the tides to be discounted. The winds duiing April, 

 moreover, were very variable, so that we may assume this particular 

 drift to have been raised from rest, especially as on the day preceding 

 that on which the batch was put out a calm prevailed in this portion of 

 the Channel. This assumption is confirmed by the fact that a bottle of 

 batch XXXVII., put out in the same place on the preceding day (April 



