MECHANICS AKD USEFUL ARTS. 35 



of paddles 21 on each wheel, and the number of revolutions taken with a 

 counter. Every other paddle on each wheel was then removed, reducing the 

 number to 1 1 (leaving two for the odd number), in their former position, and 

 one voyage was made with the wheels in this state, and the number of revo- 

 lutions was also counted. This last performance was not repeated, for the 

 concussion of the paddles on their entrance into the water was so great as to 

 cause an excessive vibration and shaking in every part of the vessel, the 

 paddles struck the water, as if acting upon a solid instead of a fluid substance. 

 " This was the more remarkable, as with 21 paddles in each wheel not the 

 slightest vibration of the hull had ever been experienced from the action of 

 the machinery, the Mississippi being noted for solidity and steadiness." "WTien 

 21 paddles were again restored to the wheels the vibrations and shakings of 

 the hull of the frigate ceased, and all was smooth, steady, and pleasant as 

 before. With the 11 paddles the frigate's wheels made 3536 revolutions m 

 the 37-J miles, while with the 21 paddles she only made 3011, that being the 

 mean of eight passages. The amount of slip with the 11 paddles was 25*74 

 percent; with the 21 paddles 12-79 per cent. The slip, therefore, of the 

 lesser number of paddles was twice as much as the greater number. The 

 paddles at the periphery of the wheel when 21 were used were 4*338 feet 

 apart; with the eleven 8 '6 7 6 feet apart. This relationship of the paddles, 

 and the different results produced by them are scientific data of very great 

 importance. 



WOOD-LINED BEARIXGS FOR SHAFTS. 



Messrs. John Penn & Sons, of England, have recently, with much success, 

 applied a novel kind of lining to the stern-bearings of the shafts of screw- 

 propellers. The journal is covered with a thin belt of brass, which forms the 

 rubbing surface of the shaft. The fixed rubbing surfaces of the bearing are 

 composed of strips or segments of lignum vitce, or other hard wood. These 

 segments of wood are fitted into dove-tail grooves in the casing, and smaU 

 spaces are left between each strip to allow the water to enter and keep the 

 rubbing surfaces cool. The segments may be made to key up when worn, by 

 being fitted into wedge-shaped grooves. This kind of bearing seems pecu- 

 liarly adapted for the situation hi which the exigences of the screw-propeller 

 have caused it to be applied. 



OX THE STRENGTH OF BOILERS. 



Mr. Fairbairn, in a recent lecture before the Manchester (England) Me- 

 chanics' Institute, made the following statements respecting the strength of 

 steam-boilers. According to Mr. F., the cylindrical or spherical boiler was 

 the most eligible and strongest form in which iron plates could resist internal 

 pressure. The deduction for loss of strength, on account of riveted joints and 

 the position of the plates, was about 30 per cent, for the double riveted joists, 

 and 44 per cent, for the single ones ; the strengths (calling the plates 100) 

 being in the ratio of 100, 70, and 56. "We had found that 34,000 Ibs. to the 



