PROPORTIONS OF ENGINES. 



she may be lost in consequence of her temporary derangement. 

 Upon the whole, therefore, the application of feathering wheels to 

 vessels intended to perform long voyages through stormy seas, 

 appears to be of doubtful propriety. For channel trips, and in 

 situations where the wheels can be carefully examined at short 

 intervals, the risk is not so great ; but in that case nearly the 

 same benefits will be attained by increasing the length of the 

 paddle-floats, and giving the wheels less dip. There is no mate- 

 rial difference between the performance of a feathering wheel and 

 that of a radial wheel, if the two wheels be of the same diameter, 

 and if they have both a light dip with long narrow floats. And, 

 as in sea-going vessels, the wheels must necessarily be of con- 

 siderable diameter, and as there is nothing to prevent the other 

 circumstances conducive to efficiency from being observed, it 

 follows that in ocean-vessels radial wheels would be about as 

 efficient as feathering wheels, but for the circumstance of a vari- 

 able immersion. It is not necessary, however, that there should 

 be much variation in the immersion if large vessels be employed, 

 or if coal is more frequently taken on board during the voyage ; 

 and as neither of these alternatives is attended with the risk 

 incident to the use of feathering wheels, they appear to be entitled 

 to that preference which ultimately they are likely to obtain. 



61. In oscillating engines the piston-rod is usually made one- 

 ninth of the diameter of the cylinder, and the crank -pin is made 

 about one-seventh of the diameter of the cylinder. The diameter 

 of the paddle-shaft must have reference not merely to the 

 diameter of the cylinder, but also to the length of the stroke of 

 the piston, or, what is the same thing, to the length of the crank. 

 If the square of the diameter of the cylinder in inches be multi- 

 plied by the length of the crank in inches, and the cube-root of 

 the product be extracted, then that root multiplied by '242 will 

 give the diameter proper for the shaft in inches at the smallest 

 part. The diameter of the trunnions is regulated by the diameter 

 of the steam and eduction pipes, and these are each usually about 

 one-fifth of the diameter of the cylinder ; but it is better to make 

 the steam trunnions a little less, and the eduction trunnions a 

 little more, than this proportion. The steam and eduction pipes, 

 where they enter their respective trunnions, are kept tight by a 

 packing of hemp, which is compressed by a suitable ring or 

 gland, tightened by screws. In land engines the air-pump and 

 condenser are each made about one-eighth of the capacity of the 

 cylinder, but in marine engines they are made somewhat larger. 



62. Submerged propellers, whatever be their form, are exempt 

 from many of the disadvantages which are common to every 

 species of paddle-wheel. It will be evident that the effect of 



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