THE INCLINED PLANE, WEDGE, AND SCREW. 



287 



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. 5. The theoretical estimation of the power of 



Fig. 5. 



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 of- 

 fered by the cohesion of bodies, that they admit of no numerical comparison. 

 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 invaria- 

 i bly urged by percussion, while the resistances which it has to overcome are 

 as constantly forces of the other kind. Although, however, no exact numeri- 

 cal 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. 



In the arts and manufactures, wedges are used where enormous force is to 

 be exerted through a very small space. Thus it is resorted to for splitting 

 masses of timber or stone. Ships are raised in docks by wedges driven under 

 their keels. The wedge is the principal agent in the oil-mill. The seeds 

 from which the oil is to be extracted are introduced into hair bags, and placed 

 between planes of hard wood. Wedges inserted between the bags are driven 

 by allowing heavy beams to fall on them. The pressure thus excited is so in- 

 tense, that the seeds in the bags are formed into a mass nearly as solid as 

 wood. 



Instances have occurred in which the wedge has been used to restore a tot- 

 tering edifice to its perpendicular position. All cutting and piercing instru- 

 ments, such as knives, razors, scissors, chisels, &c., nails, pins, needles, awls, 

 &c., are wedges. The angle of the wedge, in these cases, is more or less 

 acute, according to the purpose to which it is to be applied. In determining 

 this, two things are tojpe considered the mechanical power, which is in- 

 creased by diminishingTne angle of the wedge, and the strength of the tool, 

 which is always diminished by the same cause. There is, therefore, a practi- 

 cal limit to the increase of the power, and that degree of sharpness only is to 

 be given to the tool which is consistent with the strength requisite for the 

 purpose to which it is to be applied. In tools intended for cutting wood, the 

 angle is generally about 30. For iron, it is from 50 to 60 ; $ and for brass, 

 from 80 to 90. Tools which act by pressure maybe made more acute than 

 those which are driven by a blow ; and, in general, the softer and more yield- 



