26 



OF CENTRAL FORCES. 



LECT. such a force as would carry the ship through an equal 

 .^v^, 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 A EC in a minute : which line 

 (because the forces are equal and perpendicular to each 

 other) will be the diagonal of an exact square. 

 To confirm this law 



A| _ ^ ' Ktt 



E 



by an experiment, let 

 there be a wooden 

 square A BCD so 

 contrived, as to have 

 the part B EFC made 

 to draw out or push i"-pt ' " " *" CJT^ ~^ 



to the square at plea- 

 sure. To this part let the pulley H be joined, so as to turn 

 freely on an axis, which will be at H when the piece is 

 pushed in, and at h when it is drawn out. To this part let 

 the ends of a straight wire k be fixed, so as to move along 

 with it, under the pulley ; and let the ball G be made to 

 slide easily on the wire. A thread m is fixed to this ball, 

 and goes over the pulley to /; by this thread the ball 

 may be drawn up on the wire, parallel to the side A D, 

 when the part B EFC is pushed as far as it will go 

 into the square. But, if this part be drawn out, it will 

 carry the ball along with it, parallel to the bottom of the 

 square D C. By this means the ball G may either be 

 drawn perpendicularly upward by pulling the thread m, 

 or moved horizontally along by pulling out the part 

 BEFC, in equal times, and through equal spaces; 

 each power acting equally and separately upon it. But 

 if, when the ball is at G, the upper end of the thread be 

 tied to the pin /, in the corner A of the fixed square, 

 arid the moveable part B E F G be drawn out, the ball 

 will then be acted upon by both the powers together : 

 for it will be drawn up by the thread towards the top of 

 the square, and at the same time carried with its wire k 



