Objects, Advantages, And Pleasures Of Science.

them onwards at the same time that they are are drawn towards the point . The line in which a body moves while so drawn and so driven , depends upon the force it is pushed with , the direction it is pushed in , arid the kind of power that draws it

towards the point ; but at jiresent , we are chiefly to regard the latter circumstance , the attraction towards the point . If this attraction be uniform , that is , the same at all distances from the point , the body will move in a circle , if one direction be given

to the forward push . The case with which we are best acquainted is when the force decreases as the squares of the distances , from the centre or point of attraction , increase ; that is , when the force is four times less at twice the distance , nine times

less at thrice the distance , sixteen times less at four times the distance , and so on . A force of this kind acting on the body , will make it move in an oval , a parabola , or an hyperbola , according to the amount or direction of the impulse , or forward push , originally given ; and there is one

proportion of that force , which , if directed perpendicularly to the line in which the central force draws the body , will make it move round in a circle , as if it were a stone tied to a string and whirled round the hand . The most usual proportions in

nature , are those which determine bodies to move in an oval or ellipse , the curve described by means of a corcl fixed at both ends , in the way already explained . In this case , the point of attraction , the point towards which the bodis drawnwill be

y , nearer one end of the ellipse than the other , and the time the body will take to go round , compared with the time any other body would take , moving at a different distance from the same point of attraction , but drawn towards that point with

a force which-bears the same proportion to the distance , will bear a certain proportion , discovered by mathematicians , to the average distances of the two bodies from the point of common attraction . If you multiply the numbers expressing the times of going round , each by itself , the Products will be to one another in the

proportion of the average distances multiplied each by itself , and that product agahi by the distance . Thus , if one bod y take two hours , and is five yards distant , the other , being ten yards off , will take

something less than five hours and forty minutes . * Now , this is one of the most important truths in the whole compass of science , for it does so happen , that the force with which bodies fall towards the earth , or what is called their gravity , the power that draws or attracts them towards the earth , varies with the distance from the Earth ' s

centre , exactly in the proportion of the squares , lessening as the distance increases at two diameters from the Earth's centre , it is four times less than at one ; at three diameters , nine times less ; and so forth . It goes on . lessening , but never is destroyed , even at the greatest distances to which we can reach by our observations , and there can be no rlonbt ' of its extending

indefinitely beyond . But by astronomical observations made upon the motion of the heavenly bodies , upon that of the moon for instance , it is proved that her movement is slower and quicker at different parts of her course , in the same manner as a body ' s motion on the earth would be

slower and quicker , according to its distance from the point it was drawn towards , provided it was drawn , by a force acting in the proportion to the squares of the distance , which we have frequently mentioned ; and the jiroportion of the time

to the distance is also observed to agree with the rule above referred to . Therefore , she is shown to be attracted towards the Earth by a force that varies _ according to the same proportion in which gravity varies ; and she must consequently move in an ellipse round the earth , which is placed in a point nearer the one end than the other of that curve . In like manner .

it is shown that the earth moves round the Sim in the same curve line , and is drawn towards the Sun by a similar force ; and that all the other jilanets in their courses , at various distances , follow the same rule , moving in ellipses , and drawn towards the Sun by the same kind of

power . Three of them have moons like the earth , only more numerous , for Jupiter has four , Saturn seven , and Herschel six so very distant , that we cannot see them