460 ON THE DETERMINATION OF THE TRUE STRENGTH 
of the floats of a and the mean of both days makes it =. or almost exactly 
what Dr Rosinson 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 (4) 
might probably not obtain equally under all velocities of wind, but it has not 
been thought worth while 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 nature 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 will 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 which 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 from which the wind 
is coming. 
Under such circumstances, what will be the apparent wind, or, in other words, 




