460 ON THE DETERMINATION OF THE TRUE STRENGTH 



of the floats of 3:^ ; and the mean of hoth days makes it 3:^, or almost exactly 

 wliat Dr Robinson stated that it should be from theoretical investigation. 



The result was, so far, perfectly satisfactory, and seemed to shew that the 

 instrument was fully entitled to be tried at sea, as giving a good and convenient 

 measure of the velocity of the wind. The particular proportion mentioned (.|) 

 might probably not obtain equally under all velocities of wind, but it has not 

 been thought worthwhile to try the instrument at other velocities on shore ; because 

 there are much more powerfully modifying circumstances in the rolling motion 

 of the ship, the full effect of which can only be determined by actual experiment 

 at sea. But whatever alteration of the value of the revolutions takes place under 

 such conditions, naval officers may be assured of this, that a certain physical 

 connection between the velocity of the air and the revolutions of the anemometer 

 exists, and its exact natm^e may be easily investigated and discovered, and the 

 strength of the wind may then be entered in the log-book, in numbers expressive 

 of the velocity of the air in knots per hour, or in the same terms as the motion 

 of the vessel ; and as the direction of the wind is already sufficiently well observed 

 by the different vanes at present in use on board ship, all the elements of the 

 apparent wind may, with the assistance of this anemometer, be looked upon as 

 satisfactorily obtained. 



This apparent wind being, however, the combined effect of the motions of 

 the ship and of the air, may be very different from the true wind, both in di- 

 rection and in strength. When the ship is going with the wind, the velocity of 

 the true wind will be equal to the sum of the velocities of the ship and of the 

 apparent wind; and when going against the wind it Avill be equal to their 

 difference, without alteration of direction in either instance. But in almost 

 every other possible case both the velocity and the direction will be changed, the 

 problem being a particular application of the well known and important theorem 

 in mechanics of the parallelogram of forces ; the velocity of the ship observed 

 forming one side, the velocity of the apparent wind — also obtained from obser- 

 vation — being the diagonal, and the true wind to be determined, another side. 



This may be illustrated familiarly as follows:— Let the line A B (Plate XI.) 

 represent the motion of the ship, the length of the line shewing the velocity in 

 knots per hour as determined by the log-line, and the position of the line shewing the 

 vessel's course, or the direction to luhich the vessel is proceeding, obtained by refe- 

 rence to the vanes and the compass. Similarly, let the lines A C or B D represent 

 the true wind, or the wind which a person at rest would feel to be blowing over 

 the sea during the time that the ship passed from A to B. The length of the 

 lines A C and B D shew the velocity of the wind in knots per hour, given by the 

 anemometer, and the position of the lines gives the direction /*•(;»( which the wind 



)ming. 



Under such circumstances, what will be the apparent wind, or, in other words. 



