sistance produced by the action of the body on the road; and it is only by investigating the nature of this resistance, and its law, that the necessary qualities in the drawing or impelling power can be fully understood. As the presence of resistance on the road does not supersede the necessity of the first impulse, it follows that every mass which is to be moved, requires a much greater exertion of power at starting than subsequently; but, as this exertion is continued only for a short period. I may omit its consideration, when that purpose is to investigate the power necessary to keep it in constant action.
The power of traction necessary to sustain the progressive motion of a boat floating on a liquid arises from the resistance of the liquid lying immediately before the boat. It is necessary that the vessel should divide the fluid which lies in its way; and the force necessary to move this with the speed of the vessel must be supplied by the power of traction or impulsion, whatever that power may be. It will be sufficiently obvious, on consideration, that the quantity of liquid which is thus driven or divided before the vessel, depends, not on the whole magnitude of the vessel, but on the magnitude of the transverse section of that part of the vessel which is beneath the surface of the liquid. It is true that this conclusion requires some modification in practice, and that the shape of the vessel and other circumstances should be taken into account, in accurate calculations; but the resistance mainly depends, as above stated, on the transverse section, and may be considered, cæteris paribus, proportional to that section. Now, the more rapidly the vessel is moved, the more rapidly the liquid must be removed before it, and, therefore, the greater the force necessary to impel it in this manner; and
