162 LECTURE XVIII. 



to another, the most natural and obvious method, if they are portable, is to 

 carry them. There is, however, some scope for theory even in this common 

 operation, and we have seen that calculations have been made in order to 

 determine the most advantageous burden for a porter to carry, but the 

 experience of a few trials would in general be a better guide. Some carry 

 weights on their heads, others on their shoulders, others low down on their 

 backs ; and according to the situation of the burden, they bend forwards 

 or backwards, so that the common centre of gravity of the weight and 

 the body comes immediately or very nearly over some part of the ground 

 between their feet. The difficulty of carrying a weight at the extremity 

 of a long rod is easily understood from the properties of the lever, and 

 the same principles will enable us to determine the distribution of a load 

 between two porters, in whatever way they may carry it. Supposing the 

 weight to be placed on a porter's horse or hand barrow, and at equal dis- 

 tances from both extremities, each of the men will support an equal portion 

 of it ; but if it be nearer to the one than to the other, the load will be dis- 

 tributed in the same proportion as the poles are divided by the centre of 

 the burden. For instance, if the weight were 300 pounds, and it were 

 one foot distant from the one, and two from the other, the first would have 

 to carry 200 pounds, and the second 100. If the porters ascend a hill, or 

 a flight of steps, the distribution of the load will remain the same, provided 

 that the centre of the weight lie in the plane of the poles. But if the weight 

 consists of a large body placed on that plane, the centre of gravity being 

 above it, the effect of an inclination to the horizon may materially change 

 the distribution of the load, since the pressure will always be determined 

 by the distance of the ends of the poles from the line passing perpendicu- 

 larly through the centre of gravity ; so that if the elevation were sufficient, 

 the whole burden might rest on the lower porter. And in the same manner, 

 if the weight were suspended below the poles, the inclination would cause 

 a greater proportion of the load to be borne by the upper porter. The force 

 is, however, only thus distributed as long as the arms of the porters con- 

 tinue parallel to each other ; but the inequality would naturally be lessened 

 by a change of the directions in which they would act ; it would only be 

 necessary that those directions should meet in some part of the vertical line 

 passing through the centre of gravity ; the magnitude of each force would 

 then be determined by the length of the side of a triangle corresponding to 

 its direction, and the load might be either equally or unequally divided, 

 according to the positions of the arms. (Plate XVII. Fig. 220, 221.) 



A man can carry in general a weight four or five times as great as that 

 which he can raise continually in a vertical direction with the same velo- 

 city : so that we may consider the resistance to be overcome as a kind of 

 friction which amounts to about a fourth or a fifth of the weight. If we 

 attempted to draw a weight along a horizontal surface, the resistance of 

 the surface would often not only impede the motion, but also injure the 

 texture of the substance to be moved. This injury may, however, be 

 avoided by the interposition of a simple frame or dray, and the dray may 

 be armed with a substance subject to little friction, as with iron : the fric- 

 tion may also be somewhat further diminished by making the outline of 



