PLEASURES OP SCIENCE, 23 



motion, we can ascertain the position of a ship at sea ; for the obser- 

 vation of the sun's height at mid-day gives the latitude of the place, 

 that is, its distance from the equinoctial or equator, the line passing 

 through the middle of the earth's surface ; and these tables, with the 

 observations of the satellites, or moons, give the distance east and 

 west of the observatory for which the tables are calculated ; what is 

 called the longitude of the place : consequently the mariner can thus 

 tell nearly in what part of the ocean he is, how far he has sailed from 

 his port of departure, and how far he must sail, and in what direction, 

 to gain the port of his destination. The advantage of this knowledge 

 is therefore manifest in the common affairs of life ; but it sinks into 

 insignificance compared with the vast extent of those views which the 

 contemplations of the science afford, of numberless worlds filling the 

 immensity of space, and all kept in their places, and adjusted in their 

 prodigious motions by the same simple principle, under the guidance 

 of an all-wise and all-powerful Creator. 



We have been considering the application of Dynamics to the 

 motions of the heavenly bodies, which forms the science of Physical 

 Astronomy. The application of Dynamics to the calculation, produc- 

 tion, and direction of motion, forms the science of Mechanics, some- 

 times called Practical Mechanics, to distinguish it from the more 

 general use of the word, which comprehends every thing that relates to 

 motion and force. The fundamental principle of the science upon 

 which it mainly depends, flows immediately from a property of the 

 circle already mentioned, and which, perhaps, appeared at the moment 

 of little value, that the lengths of circles are in proportion to their 

 diameters. Observe how, upon this simple truth, nearly the whole of 

 those contrivances are built by which the power of man is increased, 

 as far as solid matter assists him in extending it; and nearly the 

 whole of those doctrines, too, by which he is enabled to explain the 

 voluntary motions of animals, as far as those depend upon their own 

 bodies. There can be nothing more instructive in showing the im- 

 portance and fruitfulness of scientific truths, however trivial and for- 

 bidding they may at first sight appear. For it is an immediate 

 consequence of this property of the circle, that if a rod of iron, or 

 beam of wood, or any other such solid material, be placed on a point 

 or pivot, so that it may move as the arms of a balance do round its 

 centre, or a see-saw board does round its prop, the two ends will 

 go through parts of circles, each proportioned to that arm of the 

 beam to which it belongs ; the two circles w r ill be equal if the pivot 

 is in the centre or middle point of the beam ; but if it is nearer 

 one end than the other, say three times, that end will go through 

 a circular space, or arch, three times shorter than the circular space 

 the other end goes through in the same time. If, then, the end of the 

 .long beam goes through three times the space, it must move with 

 three times the swiftness of the short beam's end, since both move in 

 the same time ; and therefore any force applied to the long end must 

 overcome the resistance of three times that force applied at the oppo- 

 site end, since the t\vo ends move in contrary directions; hence one 

 pound placed at the long end would balance three placed at the short 

 end. The beam we have been supposing is called a Lever, and the same 



