TRANSACTIONS OF SECTION G. 477 



This current of air is led out through a horizontal tube at the lowest part 

 of the gyro casing and tangential to its periphery ; the air blast acts horizontally. 



When the gyro-axle is horizontal — that is to say, when precession in any 

 one direction has ceased — the air-blast is symmetrically situated as regards the 

 vertical line through the suspended system, and on this account the air blast 

 may be considered as divided into two blasts, equal in speed and volume and 

 distance from the centre line, and under such conditions there is no tendency 

 for the reaction of the still air to cause any movement of the gyro-casing other 

 than a slight swinging of the suspended system in a plane coincident with the 

 plane of rotation of the gyro. If, however, the axis of the gyro is inclined from 

 the horizontal position, which is the case whenever the gyro is precessing, and 

 therefore not coinciding with the meridian, then the outlet of the air current is 

 tilted to one side of the centre line, and the reaction caused thereby is in such 

 a direction as to apply a turning moment about the vertical axis of the suspended 

 system, and under the influence of this turning moment a precession of the gyro- 

 axle takes place vertically in such a direction as to bring the axis once more 

 horizontal. 



FRIDAY, SEPTEMBER 1. 



The following Papers and Report were read : — 



1. On Electric Drives for Screw Propellers. By H. A. Mavor. 



The problems of marine engineering have until recent years been solved by the 

 application of various forms of the reciprocating steam-engine, and the form, 

 power, speed, and general arrangement of power-driven vessels have been developed 

 in connection with this means of propulsion. The advent of the steam turbine 

 and more recently of the explosive type of reciprocating engine has opened up 

 new lines of development, and in certain departments there is evidence that these 

 lines involve the use of intermediate devices between the power-producing and 

 the power-absorbing elements of the machinery. The necessity for these devices 

 arises when the properties of the propeller in respect to the best rate of revolu- 

 tion for the highest economy are incompatible with the same conditions as applied 

 to the requirements emerging from the power generator. The divergence in 

 these properties may be very small or it may be very great. If the divergence 

 be small, there is generally little or no advantage in respect of fuel economy to 

 be gained by the interposition of a transmission arrangement, with its necessary 

 mechanical or other losses ; but even in such cases it may be that the transmission 

 arrangement otherwise unnecessary may provide means of dealing with require- 

 ments which the steam turbine or internal-combustion engine are incapable of 

 meeting. For example very rapid manoeuvring at full power requires in the case 

 of the steam turbine a separate or partially separate reversing equipment, and 

 in the case of the explosive internal-combustion engine the most convenient device 

 hitherto produced is the one of compressed air in the working cylinders for pro- 

 ducing the required changes in the direction of motion. 



It is to be understood that there are many cases where the intervention of a 

 transmission arrangement does not appear to offer any advantage in fuel economv. 

 The limitations imnosed upon the designer by the beam and draught of the ship 

 and the weight to be carried may altogether exclude the use of the transmission 

 devices and necessitate the direct application of the power to the work. For 

 example, in high-sneed shallow-draufht vessels, or in vessels where the speed is 

 high relatively to the dimensions of the ship, the sacrifice in economy by running 

 at a high rate of revolution is not so great as to warrant the introduction of any 

 intermediate gear which would increase the weight of the vessel and therefore the 

 power to drive her. 



Some suggestions have been made from time to time for applying transmission 

 arrangements to vessels of the type of the Liixitania and Mmnetania, but these 

 are not in the opinion of the present writer by any means favourable cases for 

 transmission arrangements. 



The advantages of electric transmission over other competing methods may be 



