STEAM NAVIGATION. 



occurred where the end of the shaft being rendered white-hot was 

 actually welded to the steel plate against which it pressed, although 

 a stream of water was continually running over the surface in 

 contact. Various expedients have since then been proposed for 

 remedying this inconvenience. One of these was to let the end of 

 the shaft enter a tight cylinder of oil in the manner of a piston, 

 so that it would press against a liquid instead of a solid. Another 

 was to place a large collar upon the shaft which should press 

 against a number of balls or small rollers like those of a swivel- 

 bridge. Neither of these plans, however, appears to have beea 

 so successful as to get into general use, and one or other of the 

 following expedients is now generally adopted. The thrust of 

 the screw-shaft is received either upon a number of collars or a, 

 series of discs placed at the end of the shaft and resting on a 

 cistern of oil which is usually cast upon the base plate or some 

 solid part of the engine, and its end is sufficiently strong to bear 

 the thrust of the screw. Interposed, however, between the end of 

 the cistern and that of the shaft are two, three, or more discs of 

 metal, generally two inches thick, and having diameters equal to- 

 that of the shaft. A bolt passes through their centre to keep 

 them in line, but they are each free to revolve in the bolt, andi 

 where the shaft passes out of the cistern a collar of leather is 

 applied to prevent the oil from escaping. It will be obvious from 

 such an arrangement that if the end of the shaft which it presses 

 upon the discs begins to heat from undue friction, it will revolve 

 with somewhat more difficulty, and will consequently carry the 

 first disc round with it. The rubbing surfaces are therefore no- 

 longer at the end of the shaft, but at the first disc and the second 

 disc. In fact the rubbing surfaces, instead of being limited to a 

 single disc, are distributed over several. Those surfaces whiclu 

 begin to heat, and consequently to stick, will cease to rub, whereby 

 they will speedily become cool again and their efficiency conse- 

 quently be restored. (See Mr. Bourne's article on the " Screw- 

 Propeller " in the Appendix to Brande's " Dictionary of Science 

 and Art.") 



69. According to the same authority the best practical propor- 

 tion and form of screw-propellers for mercantile vessels are as 

 follows. Those of three blades are on the whole preferable. The 

 diameter should be as large as possible. When the area of the 

 circle described by the extremity of the arms of the screw has one 

 square foot for every two-and-a-half square feet in the area of 

 the midship section immersed, a very efficient performance is 

 obtained. The pitch of the screw should be equal to its diameter, 

 or perhaps a little exceed it, and the length measured parallel to 

 its shaft should be about one-sixth of a convolution. Thus, for 

 162 



