OV RAISING AND RJEMOVING WEIGHTS. 1211 



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 de- 

 termine the most advantageous burden for a porter to carry, but the experi- 

 ence 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 immedi- 

 ately or very nearly over some part of the ground between their feet. The 

 difiiculty of carrying a weight at the extremity of a long rod is easily under- 

 stood from the properties of the lever, and the same principles will enable us 

 to deteruMne the distribution of a load between two porters, in Avhatever way 

 they may carry it. Supposing the weight to be placed on a porter's horse, or 

 hand barrow, and at equal distances 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 distributed 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 re- 

 main 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 inclinatiou 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 perpendicularly 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 continue parallel to each other; but the inequality 

 wovdd 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 un- 



