Experimental Philosophy. [Lecture 15. 



Or if a boat (fig. 87) is drawn up the stream 

 by two men on the opposite banks, it will follow 

 the direction of neither exactly, but will proceed 

 directly in the middle of the stream. 



Suppose again (PL XIX. fig. 88) the body A 

 to represent a ship at sea ; and that it is driven 

 by the wind, in the right line AB, with such a 

 force as would carry it uniformly from A to B 

 in a minute : then suppose a stream or current 

 of water running in the direction AD, with such 

 a force as would carry the ship through an equal 

 space from A to D in a minute. By these two 

 forces, acting together at right angles to each 

 other, the ship will describe the line AEC in a 

 minute ; which line (because the forces are equal 

 and perpendicular to each other) will be the 

 diagonal of an exact square. 



If the acting forces are equal, but at oblique 

 angles to each other, so will the sides of the 

 parallelogram be : and the diagonal run through 

 by the moving body will be longer or shorter, 

 according as the obliquity is greater or smaller. 

 Thus, if two equal forces act conjointly upon the 

 body A 3 (fig. 89) one having a tendency to move 

 it through the space AB in the same time that 

 the other has a tendency to move it through an 

 equal space AD ; it will describe the diagonal 

 AGC in the same time that either of the single 

 forces would have caused it to describe either of 

 the sides. If one of the forces is greater than 

 the other ; then one side of the parallelogram will 



