TRANSACTIONS OF THE SECTIONS. 255 



to a vertical glass tube. Tlie pump-case and tank contaiu water. The fau of the 

 pump is rotated by the engine, the speed of which it indicates by the centrifugal 

 torce causing the water to rise in the ^Lass tube, and to indicate by its height, cor- 

 responding to marks on a scale, the speed, in revolutions per minute, at which tlie 

 engine is running. 



In ships (if war it is very desirable that the revolutions per minute shoidd be 

 visible at a glance, both to'the engineer in the engine-room and to the officer of the 

 watch on the bridge. This the author eli'ects by carrying two small pipes from the 

 engine-room to tha bridge ; one of theie pipes is connected to the centre, and_ the 

 other to the peripliery of a centrifugal pump tixed in the engine-room and driven 

 by the engine. Thepunip and pipes are full of water ; and the centrifugal force, 

 due to the rotation of the pump, creates a difference of pressure in tlie two pipes, 

 which difference is the same at any point in the two pipes, and is rendered sensible 

 in the engine-room or on the bridge by a differential pressure-gauge applied to the 

 two pipes, and capable of measuring the difference of pressure. _ By properly gra- 

 duating the dials of these gauges, the indications can be read in revolutions per 

 minute of the engine. 



Attached to this apparatus is an arrangement for indicating whether the eiigines 

 are going aliead or astern by means of a small osoillating air-pump, which pumps 

 air into a small pipe when the engines are going astern, and sucks the air out of the 

 pipe when they are going ahead. This pipe is carried to the bridge, and connected 

 to a small cylinder containing a piston which moves to one or other end of tlie 

 cylinder according as the pressure in the pipe is above or below that of the atmo- 

 sphere. This piston is attached to and moves a piece having the words " ahead " 

 and " astern " ;written on it, and causes one or other of these words to become 

 visible, as the case may be. 



On SteeritKj. By Joseph Woolley, LL.D., Vice-President of the Institution 



of Naval Architects. 



The only mode of steering considered in this paper is that in which the rudder 

 is the instrument employed, screws placed athwartships, in the bow or stern, and 

 jets of water on the principle of Ruthven's propeller, having been proposed, but 

 never seriously adopted. 



Ancient authors always assumed that, in accordance with the received theory of 

 resistance, the resistance on tlie rudder varied as if sin '■' <1>, where v is the velocity of 

 the ship and <P the angle to which the rudder is put over. Modern observations and 

 experience show that tliis law is incorrect ; that the resistance is not uniform over 

 the whole surface, but more effective at the fore than the aft part ; and the pivot- 

 balanced rudder gives reason to believe that the resistance of the foremost third is 

 equivalent to that on the aftermost two thirds of the wliole surface. It is also now 

 allowed by most competent authorities that the resistance at different angles varies 

 more as the sine than as the square of the sine of the angle. In accordance with 

 the old theory, the angle of maximum efficiency was laid down at 54 deg. 44 sec. 

 Bouguer points out that this angle is too large, and that it ought to be reduced, in 

 consequence of the change of direction of the water particles at the upper or 

 broader part of the ship ; and in a general way thinks it should be reduced by at 

 least 7 deg. or 8 deg. Euler, following in his steps, by reasoning analogous to rudi- 

 mentary stream-line principles, finds that v cos /3 is the velocity with which the 

 particles of water impinge on the rudder at the part where ^ represents the angle 

 made by the water-hne with the fore and aft lines. He then deduces for the angle 



of maximum efficiency at that height 90° -^31^, where y=co3-* - cos /3. Thus, 



if i3 be 4-5 deg., this angle is 29 deg. 9 min. But as the angle varies for different 

 water-lines, he concludes in favour of a mean angle of 48 deg., or at least 45 deg. 



There is reason to believe that not only does the putting over of a rudder produce 

 a direct resistance on its outer surface (which alone has hitherto found its way 



19* 



