58 On a u Speed Indicator for Ships' Propellors. 



angular velocity w of another body B, if a point on B at a 

 distance ¥ from its axis of rotation has the same linear 

 velocity as a point on the circumference of A. We then get 



ew= — r 



in which — is constant, and thus o> varies as /. 

 r 



The condition is evidently satisfied by a disk pressing on 

 the surface of a cone revolving at a uniform speed ; for the 

 linear velocity of the point in the perimeter of the disk which 

 is in contact with the cone, being identical with that of the 

 corresponding point of the cone, will be proportional to the 

 radius of the corresponding circular section of the cone, and 

 consequently to the distance of the point of contact from the 

 apex of the cone. If, further, the disk forms the nut of a screw 

 rotating in the opposite direction to the cone, and the axis of 

 the screw is placed parallel to the side of the cone, it is obvious 

 that the nut and screw must have the same angular velocity for 

 the nut to be at rest on the screw ; and the disk will therefore 

 take up a different position on the screw as the velocity of 

 the latter changes in value. 



In the " Blythswood Speed Indicator " the screw is driven 

 by the engine or shaft whose speed is to be indicated, while 

 the cone has a constant speed communicated to it by clock- 

 work. A scale placed under the screw shows the speed of 

 the nut and screw when the nut is at rest ; and by a series 

 of electric contact-pieces attached to a cable, the indications 

 can be transferred to dials placed at any distance from the 

 machine. 



When the engines reverse, a clutch keeps the screw revolving 

 always in the same direction, and at the same time pulls over 

 a commutator placed in the electric circuit, so that the dials 

 show not only the speed, but also the direction of motion, 

 ahead or astern. 



The clock is wound by the engine, and stops when the 

 machinery is at rest ; and having a carefully balanced friction- 

 governor, it is independent of gravity, and therefore keeps its 

 time under all conditions of wind and weather. 



We propose to add to the instrument a drum carrying 

 prepared paper, upon which as it revolves a pencil will 

 continuously trace the position of the nut, and thus keep a 

 record of tbe performance of the engine for future reference. 



The same principle can evidently be applied to a comparison 

 of the speeds of two engines or rotating shafts, where both 



