TRANSACTIONS OF THE SECTIONS. 



171 



On comparing these experiments, it will be seen that, although the propeller is of 

 larger dimensions than that of Mr. Apsey, the thrusts are not so great. In the first 

 case, the velocity is nearly twice as great, while the area, taken as discs, are as 

 1 : 2'47 ; but taking the respective thrusts of the two propellers, Mr. Apsey's in the 

 boiler, and mine in the open river Thames, the ratios of resistance or thrusts of the 

 propellers at one, two, and three feet immersion respectively, are not very dis- 

 similar. 



In both cases the influence of velocity is much greater than depth, and is such 

 as to approximate the action of a screw in a solid, like which the water becomes 

 when rapidly acted upon ; but the joint influence of depth and velocity shows that 

 the thrust or resistance of the screw is 6J times greater when immersed three feet 



Fig. 3. 



below the water level than when working at a level ; consequently, a screw whose 

 disc area is one-sixth, three-fourths of the area of the screw, when the level of the 

 water is level with its circumference, is equally effective. If this be the fact, as the 

 often-repeated experiments proved, it is reasonable to expect very important results 

 hereafter in the use of the screw ; and further, if one small screw proportioned as 

 above shown be as effective as one large screw working in the dead wood, how much 

 smaller and more effective will be two screws, when applied to a vessel's quarters on 

 either side of the dead wood and stern ! 



