MECHANICS AND USEFUL ARTS. 35 



As far as human ingenuity and skill can foresee, the former danger has been 

 provided against, and the apparatus forms the most ponderous system of 

 check tackle ever constructed. To the centre of each cradle is fastened the 

 iron sheave to which the check tackle is attached, weighing five tons. 

 Wrought iron chains of the largest size connect these with two other 

 sheaves, each secured to a drum, which pays out the chain and regulates 

 the whole operation. These drums and the framework on which they rest 

 having to bear the strain of the whole mass in motion, extraordinary precau- 

 tions have been taken to render them as massive as they could be made by 

 any known combination of wood and iron. The axles are formed of beams 

 of timber and strips of wrought iron bound together, forming a drum twenty 

 feet long and nine feet in diameter. The discs are solid iron, sixteen feet 

 in diameter, and weighing upwards of twenty tons, so that the weight of each 

 drum is more than sixty tons. The axle is set in an iron frame, and round 

 its outer edge passes a band of wrought iron, to work in the manner of a 

 break, which, with the aid of strong iron levers, twenty feet long, brings such 

 a pressure upon the drum as to lessen its revolutions, or entirely stop them, in 

 case the chain is being paid out too fast. Our readers may naturally ask 

 what holds the drums themselves ? The frame in which the work is set is a 

 solid piece of timber, formed by driving piles forty feet in length, and going 

 down to the gravel. The whole is bound together with iron, and strong 

 shores pass from the piles to the bed of piles on which the ways are con- 

 structed ; so that, whatever the strain, it would be impossible for the setting 

 of the drums to give way unless the river bank gave way with it. 



These are the appliances for preventing the monster running down too 

 fast '. but a powerful apparatus has been devised to act in a contrary man- 

 ner, namely : to pull her off the ways in case of her sticking fast on them 

 through any unforeseen cont/r tempts. For this purpose four lighters are 

 moored about 100 yards from the shore, fitted with crabs and sheave.-. Each 

 crab gives a strain of dxty tons, and this force of 240 tons, if necessary at 

 all, is to be applied amidships. Two lighters will also be moored at the stem 

 and two at the stern, and the chains passing to these from the ship will re- 

 turn on shore, so as to be worked with a double purchase. Stationary en- 

 gines of twenty-horse power will haul in the chains, making the whole force 

 available to pull the vessel off upwards of GOO tons. 



It only remains to add, as a matter of scientific record, that since the 

 above was written the launch of the great steamer has been effected. Ed. 

 attempted. Editor. 



ON SCREW PROPELLERS. 



The following communication on the above subject, by Prof. W. R, Hop- 

 kins, U. S. Naval Academy, is copied from Silliman's Journal : 



Is it not strange that while in heavy machinery on land revolving at high 

 velocities no difficulty is found in preventing heating in the journals, from 

 friction, that few propellers are afloat at sea th:it have not sufT'Mvd seriousV 

 from this cause? We hear of vessels on both sides of the Aihuuio, Ki<-reaa- 



