ON RAISING AND REMOVING WEIGHTS. 159 



Pullies, and their combinations in blocks, are universally employed on 

 board of ships. They are very convenient where only a moderate increase 

 of power is required ; but in order to procure a very great advantage, the 

 number of separate pullies or sheaves must be very much multiplied ; a 

 great length of rope must also be employed ; and it is said that in a pair of 

 blocks with five pullies in each, two thirds of the force are lost by the fric- 

 tion and the rigidity of the ropes. The inconvenience resulting from a 

 large number of pullies, may, however, as we have already seen, be con- 

 siderably lessened when they are arranged in Mr. Smeaton's manner,* the 

 acting rope being introduced in the middle, so as to cause no obliquity in 

 the block. Tackles, or combinations of pullies for raising weights, are 

 most conveniently supported 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 em- 

 ployed, 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 apparatus 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 scaffold- 

 ing, 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 circumstances 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 of Bridgwater's canal.1* This canal is extended, 

 above ground, for forty miles on one level : an underground navigation, 

 twelve miles long, joins it at Worsley, leading to the coal mines under 

 "Walkden moor. At a height of 35 yards above this is another subter- 

 raneous 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 canal. The inclined 

 plane is fixed in a stratum of stone, which fortunately has the most eligible 

 inclination of 1 in 4, and is 33 yards in thickness, affording the most ad- 

 vantageous means of fixing every part of the machinery with perfect 

 security. The whole length of the plane is 151 yards, besides a lock of 

 18 yards at the upper end. (Plate XVII. Fig. 216.) 



^Inclined planes are also universally employed for facilitating the ascent 



* Ph. Tr. 1751, p. 494. 



t Consult Tr. of the Soc. of Arts, xviii. 288 ; Nich. Jour. iv. 486. 



