NATURAL PHILOSOPHY. 163 



furrow-slice, it is evident that the surface of the mould-board must be 

 a warped surface, generated by a right line, moving from the position 

 of the front edge of the share, parallel to a plane director, which is per- 

 pendicular to the direction of the furrow ; the generatrix, at the same 

 time, revolving about its inner extremity, describing an angle of 135 

 or 180, according to the position hi which the furrow-slice is to be left, 

 this whole angle being described when the generatrix has moved the pre- 

 scribed length of the plough ; four feet being the maximum. The only 

 resistance to the turning over of the furrow-slice, after it is cut off, will 

 be its movement of inertia and weight. The friction occasioned by the 

 weight of the furrow-slice will impede the forward motion of the 

 plough, but forms no part of the resistance of the furrow-slice itself to 

 being turned over. But this friction is trifling, compared with that of 

 the bottom of the plough, (which depends on its weight, and not on the 

 length of the plough or the surface in contact with the ground,) and 

 still more trifling in comparison with the resistance which the solid 

 earth opposes to the cutting of the coulter and the share. The prob- 

 lem will then consist simply in determining the relation of the angular 

 movement of the generatrix to its longitudinal movement, so as to pro- 

 duce a steady and constant pressure of the mould-board upon the furrow- 

 slice till it is completely turned over into the required position. These 

 conditions will insure both the least resistance, so far as the small 

 resistance, arising from the weight and inertia of the furrow-slice, is 

 concerned, and the best performance of the work. [For the equation, 

 which expresses the relations of angular motion and time, or, in other 

 words, the angular velocity of solid bodies about fixed axes under the 

 action of various accelerating forces, see Poisson, Traite de Mecanique, 

 art. 392, of the edition of 1833.] Professor H. then displayed upon the 

 black-board an investigation involving the higher mathematical analy- 

 sis, from which he deduced the law of generation, of the surface of the 

 mould-board, and explained a practical method of constructing a model 

 from wood, in accordance with his formula, so as to fulfil the condi- 

 tions prescribed. For the purpose of producing the pulverization of 

 the furrow-slice, there must be an elevation of the mould-board at its 

 under-surface medium line. This elevation will act to the greatest 

 advantage to produce the pulverization if made upon the forward part of 

 the mould-board, where the surface is not so much warped. Experi- 

 ments might be made with an excrescence of wood, which should be cut 

 away until the degree of pulverization leaves the furrow-slice in suffi- 

 cient form to be turned over and delivered in its place without too 

 great scattering. 



ON THE "NOMINAL HORSE-POWER" OF STEAM ENGINES READ BEFORE 

 THE BRITISH ASSOCIATION FOR THE ADVANCEMENT OF SCIENCE, BY 

 L. G. HEATH. 



THE inadequacy of the present term, "nominal horse-power," for 

 giving a definite idea, either of the absolute or relative power of en- 

 gines, was first examined by comparing the engines of H. M. S. Gar- 



