MECHANICS. 



it from descending. Let A B (fig-. 10.) 

 be a plane parallel to the horizon, and A 

 1) a plane inclined to it ; and suppose the 

 whole length A D to be four times as 

 great as the perpendicular 1) B. In this 

 case the cylinder E will be supported up- 

 on the plane I) A ; and kept from rolling 1 , 

 by a power equal to a fourth part of tlie 

 weight of the cylinder ; therefore a 

 weight may be rolled up this inclined 

 plane, by a third part of the power which 

 would be sufficient to draw it up by the 

 side of an upright wall. It must also be 

 evident, that the less the angle of eleva- 

 tion, or the gentler the ascent is, the 

 greater will be the weight which a given 

 power can draw up ; for the steeper the 

 inclined plane is, the less does it support 

 of the weight; and the greater the ten- 

 dency which the weight has to roll ; con- 

 sequently, the more difficult for the pow- 

 er to support it : the advantage gained by 

 this mechanical power, therefore, is as 

 great as its length exceeds its perpendi- 

 cular height. To the inclined plane may 

 be reduced all hatchets, chisels, and other 

 edge-to 



The inclined plane, when combined 

 with other machinery, is often of great 

 use in the elevation of weights: it has 

 been likewise made use of in the late Uuke 

 of Bridge water's canal. After Ihis canal 

 has extended about 40 miles on the same 

 level, it is joined to a subterraneous na- 

 vigation about 12 miles long, by means of 

 an inclined plane, and this subterraneous 

 portion is again connected by an inclined 

 plane with another portion 100 feet above 

 jt. This plane is a stratum of stone which 

 slopes one foot in four, and is about 450 

 feet long. The boats are conveyed from 

 one level to another by means of a wind- 

 lass, so that a loaded boat descending 

 along the plane turns the axis of the wind- 

 lass, and raises an empty boat. 



The fifth mechanical power or machine 

 is the wedge ; which mav be considered 

 as two equally inclined planes, joined to- 

 gether at their bases ; then D G (fig. 21.) 

 is the whole thickness of the wedge at its 

 back ABGD, where the power is applied ; 

 KF is the depth or height of the wedge ; 

 BF the length of one of its sides ; and OF 

 is its sharp eclg-e, which is entered into 

 the wood intended to be split, by the 

 force of a hammer or mallet striking per- 

 pendicularly on its back. Thus, AB (fig. 

 22.) is a wedge driven into the cleft CED 

 of the wood FG. When the wood does 

 not cleave at any distance before the 

 wedge, there will be an equilibrium be- 

 tween the power impelling the wedge 

 downward and the resistance of the wood 



acting against the two sides of the wedge, 

 when the power is to the resistance as 

 half the thickness of the wedge at ii:> back 

 is to the length of either of its sides ; be- 

 cause the resistance then acts perpendi- 

 cularly to the sides of the wedge. But 

 when the resistance on each side acts pa- 

 rallel to the back, the power that balances 

 the resistances on both sides will be, as 

 the length of the whole back of the wedge 

 is to double its perpendicular height. 



When the wood cleaves at any distance 

 before the wedge (as it generally does) 

 the power impelling the wedge will not 

 be to the resistance of the wood as the 

 length on the back of the wedge is to the 

 length of both its sides, but as half the 

 length of the back is to the length of ei- 

 ther side of the cleft, estimated from the 

 top or acting part of the wedge. For, if 

 we suppose the wedge to be lengthened 

 down from the top CE, to the bottom of the 

 cleft at D, the same proportion will hold ; 

 namely, that the power will be to the re- 

 sistance as half the length of the back of 

 the wedge is to the length of either of its 

 sides : or, which amounts to the same 

 thing, as the whole length of the back is 

 to the length of both the sides. The 

 wedge is a very great mechanical power, 

 since not only wood, but even rocks, can 

 be split by it ; which it would be impossi- 

 ble to efVect by the lever, wheel, and 

 axle, or pulley ; for the force of the blow, 

 or stroke, shakes the cohering parts, and 

 thereby makes them separate more ea- 

 sily. 



The sixth and last mechanical power is 

 the screw ; which cannot properly be 

 called a simple machine, because it is ne- 

 ver used without the application of a le- 

 ver or winch to assist in turning it ; and 

 then it becomes a compound engine of a 

 very great force, either in pressing the 

 parts of bodies closer together, or in rais- 

 ing great weights. It may be conceived 

 to be made by cutting a piece of paper, 

 AUC (fig. 23.) into the form of an inclin- 

 ed plane or half wedge ; and then wrap- 

 ping it round a cylinder (fig. 24.) the edge 

 of the paper AC will form a spiral line 

 round the cylinder, which will give the 

 thread of the screw. It being evident 

 that the winch must turn the cylinder 

 once round, before the weight of resist- 

 ance can be moved from one spiral wind- 

 ing to another, as from d to c ; therefore, 

 as much as the circumference of a circle 

 described by the handle of the winch is 

 greater than the interval or distance be- 

 tween the spirals, so much is the force of 

 the screw. Thus, supposing the distance 

 of the spirals to be half" an inch, and the 



