VOL. XXXVI.] PHILOSOPHICAL TRANSACTIONS. 371 



Where goods are to be raised liigh, as in unloading vessels, and also to be 

 let down deep, as in loading them; if the weights do not exceed 2 or 3 ton, 

 and many hands are not to be had, then an endless screw turned by a handle 

 at each end (in an opposite situation, or with one handle and a balance to it) 

 leading an axis in perilrochio, or as it is commonly called, a worm and wheel 

 applied to a crane, with a gibbet, is most useful: for the teeth of the wheel 

 are pulled by the weight so directly against the thread of the worm in its en- 

 deavour to descend, that one- may leave the handle in any position, where it 

 will stop, without any catch, or the least danger of the weight falling 

 back again. 



But then, if you would have the weight to be let down, to descend pretty 

 quick, which cannot be performed by applying the hand to the handle, which 

 goes through a great space in comparison to the space described by the weight, 

 only give the handle a swing, and if the worm be well oiled, the handle and 

 its counterpoise, or the two handles, will perform the office of a fly in the 

 common jack, turning very fast round, and regulating the motion of the 

 weight, which from that impulse will descend continually, and not too fast, like 

 the weight of a jack. 



The way to stop this motion at any time, is to grasp the axis of the screw 

 hard, between the screw and the handle in its round part. The hand is suf- 

 ficient to do it, and will stop it in 2 or 3 turns. 



The worst cranes are those where men walk in a large wheel, by reason of 

 accidents that happen daily on account of the short space between a man's two 

 feet. This may be prevented by using quadrupeds, the length of whose 

 bodies, makes a base of sufficient length to keep the wheel from running 

 back, fig. 7. 



An Explanation of the Figures. — Fig. 1, representing a fixed crane with a 

 gibbet moving on an upright shaft or axis. Aaa, the roof of the crane, to 

 preserve the rope RTr from the weather, when the arm of the gibbet vcg being 

 turned towards y is brought under it. at, the upper piece of the crane, in 

 a horizontal position, called the plate of the crane, x, y, z, the three crane 

 posts, braced at top and bottom, ds, mn, ie, three cills within the stone 

 work, braced with wood, and made fast with an upright plate of iron, pinned 

 to the wood on each side. When the crane is not in stone work, the three 

 cills must be all in one piece, reaching from d to e. hi, hE, are the braces 

 of the main post, which come up above the level of the wharf lwb, which are 

 longer and stronger than the others. Here a cross piece, whose section is &, 

 keeps the main post from twisting. 



Ro, the capstan, or shaft of the crane, to receive the rope or chain ; which 

 3 8-2 



