It is not necessary, for the effect just described, that the inclined planes should, as represented in the figure, form an angle with each other. They may be parallel, or in any other position, the rope being carried over a sufficient number of wheels placed so as to give it the necessary deflection. This method of moving loads is frequently applied in great public works where rail-roads are used. Loaded waggons descend one inclined plane, while other waggons, either empty or so loaded as to permit the descent of those with which they are connected, are drawn up the other. (290.) In the application of the inclined plane which we have hitherto noticed, the machine itself is supposed to be fixed in its position, while the weight or load is moved upon it. But it frequently happens that resistances are to be overcome which do not admit of being thus moved. In such cases, instead of moving the load upon the planes, the plane is to be moved under or against the load. Let D E, _fig. 132._, be a heavy beam secured in a vertical position between guides F G and H I, so that it is free to move upwards and downwards, but not laterally. Let A B C be an inclined plane, the extremity of which is placed beneath the end of the beam. A force applied to the back of this plane A C, in the direction C B, will urge the plane under the beam so as to raise the beam to the position represented in _fig. 133._ Thus, while the inclined plane is moved through the distance C B, the beam is raised through the height C A. (291.) When the inclined plane is applied in this manner, it is called a _wedge_. And if the power applied to the back were a continued pressure, its proportion to the weight would be that of A C to C B. It follows, therefore, that the more acute the angle B is, the more powerful will be the wedge. In some cases, the wedge is formed of two inclined planes, placed base to base, as represented in _fig. 134._ The theoretical estimation of the power of this machine is not applicable in practice with any degree of accuracy. This is in part owing to the enormous proportion which the friction in most cases bears to the theoretical value of the power, but still more to the nature of the power generally used. The force of a blow is of a nature so wholly different from continued forces, such as the pressure of weights, or the resistance offered by the cohesion of bodies, that it admits of no numerical comparison with them. Hence we cannot properly state the proportion which the force of a blow bears to the amount of a weight or resistance. The wedge is almost invariably urged by percussion; while the resistances which it has to overcome are as constantly forces of the other kind. Although, however, no exact numerical comparison can be made, yet it may be stated in a general way that the wedge is more and more powerful as its angle is more acute. [Illustration: _C. Varley, del._ _H. Adlard, sc._ _London, Pubd. by Longman & Co._]