48 
MODE OF COMMUNICATING MOTION 
of the axis of one of the wheels moveable, and to obtain the pressure by the 
elasticity of a spring somewhat in the manner represented at fig. 35. 
Fig. 35. 
In this case, however, the loss by friction must be considerable, since both 
the axes have to endure the pressure necessary to ensure sufficient contact, 
which, in order to be secure from slipping, ought to exceed one-sixth of the 
whole power. The wheel upon the screw axis must also necessarily be much 
less than the diameter of the screw, as otherwise the driving wheel would be 
too large for the limits of a steam vessel. If we suppose the circle representing 
the line of mean resistance of the screw to be twice the diameter of the wheel 
upon the screw axis, we should require a pressure at the points of contact more 
than one-third of the power necessary to turn the screw, which would therefore 
generate a friction upon the two axes in the proportion which the diameters of 
the wheels are to those of their shafts. 
It has been proposed to turn the screw by the combined action of bands and 
tangential pressure; this arrangement, however, must clearly be liable to the 
objections we have made to the bands. 
The writer here submits a contrivance of his own by which tangential pressure 
may be obtained with a comparatively small amount of axis friction. 
In this arrangement, shown at fig. 36, it is proposed to use two engines of the 
form of which stationary engines are usually constructed, that is to say, they would 
have their beams above the cylinders and cranks: d d are the cranks, which are at 
a right angle to each other; c c the intermediate shaft; bab is a double coned 
wheel connected by the cylindric part a. This is all cast in one piece, and accu 
rately turned on the conical parts bb; e g and/*h are two wheels attached to axes