MAC 



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other animals, weights, springs, the wind, 

 a stream of water, &c. 2. The velocity 

 of this power, or the space it moves over 

 in a given time. 3. The resistance, or 

 quantity of the weight to be moved. 4. 

 The velocity of this weight, or the space 

 it moves over in the same given time. 



The two first of these quantities are al- 

 ways in the reciprocal proportion of the 

 two last : that is, the product of the first 

 two must always be equal to that of the 

 last : hence, three of these quantities 

 being given, it is easy to find the fourth ; 

 for example, if the quantity of the power 

 be 4, its velocity 15, and the velocity of 

 the weight 2, then the resistance, or quan- 

 tity of the weight, will be equal to 

 4 X 15 60 

 ~T~ ~~2 ~ 



Compound machines are extremely nu- 

 merous, as mills, pumps, wheel-carriages, 

 clocks, fire-engines, &c. See ENGINE, 

 MILL, Pu^rp, WATER worfrs, &c. 



Machine denotes any thing that serves 

 to augment or regulate moving powers, 

 or it is a body designed to produce mo- 

 tion, so as to save either time or force. 

 Machines are either simple or compound. 

 The simple machines ave the mechanical 

 powers, viz. the lever, the wheel and 

 axis, the pulley, the inclined plane, the 

 wedge, and the screw. See MECHANICS' 



These simple machines serve for dif- 

 ferent purposes, and it is the business of 

 the skilful mechanician *o select and com- 

 bine them in such a manner, as may be 

 best adapted to produce the effect of 

 which he stands in need. Compound 

 machines are formed from these simple 

 ones. These may be indefinitely varied, 

 and they belong to all the branches of 

 science. Descriptions of many of the 

 most useful, and which serve to exhibit 

 the principles of machinery, will be 

 found in various parts of our work. See 

 ENGINE, HYDRAULICS, PNEUMATICS, 

 &c &c. 



The modes of applying mechanical 

 forces are almost as various as the ma- 

 chines that are constructed, and the pur- 

 poses for which they are employed. In 

 general the human strength is applied by 

 means of levers, or winches, or by walk- 

 ing wheels, which slide beneath them as 

 they attempt to ascend. The force of 

 other animals is applied by a horizontal 

 arm projecting from a vertical axis, to 

 which they are harnessed. When mo- 

 tion is simply communicated to a sub- 

 stance placed before the moving body, 

 such materials are used as are capable of 

 exerting a repulsive force ; but when the 



body to be moved is behind the moving 

 power, and is pulled along with it, chains 

 or ropes are sometimes more convenient. 

 When the direction of motion communi- 

 cated is also to be changed, levers or 

 cranks may be employed, united by joints 

 or hinges of various kinds. Sometimes a 

 long series of connected rods is suspend- 

 ed by other rods or chains, so as to con- 

 vey the effect of the force to a considera- 

 ble distance ; in this case the motion is 

 generally alternate, as when pumps are 

 worked by means of a water wheel at a 

 distance from the shafts in which the 

 pumps are placed. For the communica- 

 tion of a rotatory motion, Dr Hooke's 

 universal joint, formed by a cross, mak- 

 ing the diameters of two semicircles, one 

 of which is fixed at the end of tach axis, 

 is frequently used. The best mode of 

 connecting a rotatory motion with an al- 

 ternate one is, in all common cases, to 

 employ a crank, acting on one end of a 

 long rod which has a joint at the other. 

 If the rotatory motion of the crank be 

 equable, the progressive motion of the 

 rod will be gradually accelerated and re- 

 tarded, and for a considerable part of the 

 revolution the force exerted will be near- 

 ly uniform. The force applied to a ma- 

 chine may, in general, be divided into two 

 portions, the one employed in opposing 

 another force, so as to produce equili- 

 brium only, the other in generating 1 mo- 

 mentum. With respect to tlu- first por- 

 tion, a single crank has the inconvenience 

 of changing continually the mechanical 

 advantage of the machinery ; with regard 

 to the second, its motion in the second 

 quarter of its revolution is accelerated, 

 instead of being retarded, by the inertias 

 which this portion of the force is intended 

 to overcome ; hence the motion is irregu- 

 lar. This difficulty may be remedied by 

 employing cranks in pairs, one of which 

 being fixed so as to make a right angle 

 with the other, which is moreover the best 

 position for two winches to be turned by 

 two labourers; since the point of the 

 circle, in which a man can exert his 

 greatest strength, is nearly at the dis- 

 tance of a right angle, or a little more, 

 from the point at which his force is 

 smallest. But of all the modes of com- 

 municating motion, the most extensively 

 useful is employment of wheel-work, 

 which is capable of varying its direction 

 and its velocity without any limit. See 



1VJ ACHINE, electric. The electric ma- 

 chine consists of three parts, the electric 

 body, which is rubbed ; the rubber, which 



