ON RAISING AND REMOVING WEIGHTS. 207 



ligidity of the ropes. The inconvenience resulting from a large number of 

 puUies, may, however, as we have already seen, be considerably lessened 

 when they are arranged in Mr. Smeaton's manner, the acting rope being in- 

 troduced in the middle, so as to cause no obliquity in the block. Tackles, 

 or combinations of pullies for raising weights, are most conveniently sup- 

 ported on shore by means of shears, which consist of three rods, or poles, 

 resting on the ground, and meeting each other in the point of suspension. 

 For raising stones in building, two poles are employed, with a rope fixed to 

 their summit, which keeps them in a proper position ; their lower ends are 

 usually connected by a third pole, which serves as an axis. (Plate IV. Fig. 56. 

 Plate XVII. Fig. 214.) 



Sometimes a pulley is drawn horizontally along a frame, setting out from 

 the point where the rope is fixed, so that while the bucket is raised, it is 

 also transferred diagonally to the opposite end of the scaffolding. This ap- 

 paratus is used in some of the Cornish stream works, in which the earth of a 

 whole valley is raised, in order to be washed for the separation of tin ore. 

 (Plate XVII. Fig. 215.) 



A fixed inclined plane is often of use in assisting the elevation of great 

 weights, by means of other machinery. It is supposed that in all the edifices 

 of remote antiquity, where great masses of stone were employed, as in the 

 pyramids of Egypt, and the druidical temples of this country, these vast 

 blocks were elevated on inclined planes of earth, or of scaffolding, with the 

 assistance also of levers and rollers. Inclined planes are frequently used for 

 drawing boats out of one canal into another; and sometimes the local cir- 

 cumstances are such that this may be done with great convenience, merely 

 by allowing a loaded boat to descend, and to turn the axis which raises an 

 empty one. An example of this may be seen, on a large scale, in the Duke 

 cf Bridgwater's canal. This canal is extended, above ground, for forty 

 miles on one level; an underground navigation twelve miles long joins it at 

 Worsiey, leading to the coal mines under VValkden moor. At a height of 

 354- yards above this, is another subterraneous portion, nearly six miles in 

 length. The connection between these levels is formed by an inclined plane: 

 the boats are let down loaded, and proceed three miles along the tunnel into 

 the open cani;il. The inclined plane is fixed in a stratum of stone, which 



