same cause.

plodes impulse altogether, as a cause of motion; and resolves every kind of motion into pressure. If his unintelligible jargon in that chapter mean any thing, we are there given to understand · that the motion of a falling stone, and a stone thrown by the hand, proceeds from one and the And Mr. Nicholson in the passage before quoted says, that the motion of a ship upon the surface of the sea, is regulated by the same cause as that of a stone or any other solid body descending through a fluid by the force of gravity, viz. by this celebrated principle of the conservatio virium vivarum. But the motion of a body descending through a fluid by the force of gravity, and that of a ship or any other body upon the surface of the earth, are very different, and proceed from very different causes.

The primary cause of the motion of a body descending through fluids by the force of gravity, arises from the influence of that great principle of attraction which the Creator has placed in the centre of the earth; and the particular degree of velocity with which it moves, is regulated by the condensing influence of that principle of attraction which is placed in its own body; or, in other words, by the difference of

the specific gravity of its own body, above that of the fluid through which it moves. These two powers accompany the body throughout the whole of its descent, and are constantly impelling it forward with the same uniform force, however great its velocity may be. The motion of any body upon the surface of the earth, is, on the contrary, always caused by the action of some other external body upon it; and the velocity with which it moves, is always in proportion to the velocity of the motion of the impelling body.

Without having recourse, as Mr. Leslie says, to invisible agents, and without the aid of the higher geometry, we can easily explain the cause of that acceleration in the motion of the ship, for a short space after she first gets under way, which Mr. Nicholson speaks of, and also the cause of that equality of motion which she then acquires. And for this purpose,

Let the line A B, Fig. 3, represent the current of the wind; and C a ship just getting under way, to go directly before the wind. As the force of the wind to impel the ship forward, by means of the sails, is always proportionable to its velocity, let us suppose the velocity of the wind to be represented by the number 6, or

by any other number greater than that. Let us suppose, that the ship being impelled forward by the force of the wind acting upon its sails, has, when it arrived at the point a, acquired a velocity equal to 1; at b, a velocity of 2; at c, of 3; at d, of 4; and so on; its velocity increasing in arithmetical progression. Now, as the force of the wind consists entirely in its velocity, it is obvious, that every portion of ve locity which the ship acquires, will destroy a proportional part of the force of the wind; and the force of the wind upon the sails will be at all times exactly proportioned to the difference between the velocity of the wind, and that of the ship. After the sails are spread, and before the ship begins to move, the wind acts upon the sails with its full force, which has been supposed to be equal to 6. But, when the ship begins to move forward, and has arrived at the point a, where it is supposed to have acquired a velocity of 1, equal to one-sixth part of the velocity of the wind, it is obvious, that the wind can then only act upon the sails of the ship with a force equal to five-sixths of its original power. When it arrives at b, and acquires there a velocity of 2, another sixth part of the force of the wind would be there destroyed; and when it arrived

at c, and acquired there à velocity of 3, equal to one half of the velocity of the wind, it could then act upon the sails of the ship with only one half of its original power. The ship will

still go on increasing in velocity, until the force of the wind, proportional to the difference between its own velocity and that of the ship, is no more than sufficient to overcome the resistance of the water, when it will proceed with uniform velocity. But if the wind should either increase or decrease in velocity, the velocity of the ship would immediately suffer a similar alteration; until the force proportional to the difference between the velocity of the two moving bodies is reduced to an equality with the resisting force. It makes no sort of difference, whether the ship sails directly before the wind, or in an oblique course: the force of the wind will be still reduced in the same manner by the velocity of the ship. That equality which we observe in the motions of wind mills, water-mills, carriages drawn by horses, or pushed forward by men, all proceed from the same cause, viz. from the reduction of the impelling force by the velocity of the body which is thereby put in motion. For example, if it was possible for a man, in pushing a wheelᏞ

barrow before him, to increase the velocity of his own motion to that degree which would enable him to exert constantly the same uniform force to push the barrow forward, which he did at first to put it in motion, this original force being thus continued, it would produce precisely the same effect as the force of gravity acting upon the leaden ball in its descent through the air, and the velocity of the barrow, like that of the leaden ball, would be continually increasing.

But let us again inquire into the effects which this pretended principle must necessarily have produced upon the motion of the ship, if it had been a real principle of nature, and really possessed the power of obstructing its motion in proportion to the squares of its velocity.

The velocity of the wind has already been supposed to be represented by the number 6, and the increasing velocity of the ship after it was at first put in motion, and when it arrived at the points a, b, c, to be represented by the numbers 1, 2, 3, (see Plate II. No. 2.) As this pretended principle is said to obstruct the motion of bodies in proportion to the square of their velocity, the square of these numbers is 1, 4, 9, as the velocity of the wind, and consequently its force is only 6; it is obvious that the ship could