io6 Sir I S A A C. N E W T O N’s Book II,

represented by 5, and the body b, represented by 3, moveswith a velocity represented by 4 ; then the quantity of motionof Ay shall be to the quantity os motion of b, in the com-pounded ratio of 2 to 3 and os 5 to 4, that is as 2 x 5 to 3 x 4,or as 10 to 1 2. There appears to be no ground for making adistinction between the quantity of motioji and the force of abody in motion ; as all the power or activity of body arises fromand depends upon its motion. We are not, however, to ex-pect that all the effects of the motion of bodies should be pro-portional to the quantity of motion, unless^ due regard be hadto the time of the motion, and to the direction in which itacts, according to the true principles of mechanics. A body,in consequence of its uniform motion, describes a certain spacein a certain time ; but there is no space so great that may notbe described by it, if the time be not limited. When a bodyacts upon another body, the effect is very different according tothe direction in which it acts. How necestary it is to have re-gard to these, in determining the effects of the motions andactions of bodies, will appear more fully in the next chapter.

13. When a body tends to move, but is hindered by some- obstacle, this tendency is called pressure. It is not to be com-pared with the force of a body in motion, no more than aline is to be compared with the rectangle that is generated byit. Of this kind is the gravity of a body that rests and pressesupon a table, or of water upon the bottom of a vessel, or ofair upon the fails of a ship. When the obstacle is removed,the continual action of the pressure generates motion in thebody, in any finite time. Thus gravity accelerates the motionof falling bodies, by acting incessantly upon them. When anorifice is opened in the bottom of a vessel, the pressure of thestuid accelerates the motion of the issuing water, and, in anexceeding little time, brings its velocity to a height. When