the South African Institution. 107 



memoir contains the description of a revolving paddle proposed 

 Jby Cant. Pole, to be applied to steam-ships. It was accom- 

 panied by additional observations from Capt. Pole, in which 

 lie proposed and described a species of paddle acting by direct 

 impact, to be employed for the same purpose. Dr. Adamson 

 elucidated this subject by remarks of the following tenor ; be- 

 ing chiefly an outline of a paper on the subject, prepared by 

 him about ID years ago. 



Before being able to investigate theoretically the effects of 

 different kinds of machinery, we must derive from our ex- 

 perience some measure according to which the power we seek 

 to create may be estimated. This measure is attained with 

 accuracy sufficient for practical purposes, in the case of the 

 steam-boat, by assuming the stream of water driven off from 

 the vessel as a representative of the force by which it is pro- 

 pelled. We thus simplify the problems off red us, by identify- 

 ing them, with the determination of the effect of streams of 

 given sections and velocities. We may assume the resistance 

 of the vessel to be constant, or that the different kinds of 

 machinery proposed are applied to the same vessel : and we 

 have to consider what are the conditions under which they will 

 communicate to it a given velocity. The stream or efflux re- 

 quired for this end, may have different charaetets; as being 

 either of a large section and small velocity, or 1 of smaller section 

 and great velocity: and if we fix en a determined speed to be 

 given to the vessel, it is easy to assign theoretically, the most 

 advantageous, velocity of the stream, or of the propeller which 

 creates it. There can be little doubt that this velocity has 

 hitherto been rated too low; and that with the common ex- 

 ternal machinery the velocity might be advantageously in- 

 creased, and the section of the stream or size of the propelling 

 parts diminished. The present cetablished relations between 

 these elements, seem to arisj, not so much from the erroneous 

 theories which have propagated on the subject, as from the 

 difficulty of increasing the velocity to its requisite value, with- 

 out producing the greater disadvantage of adding complexity 

 to the internal machinery. The experiments necessary for the 

 proper elucidation of these matters could be easily made, and 

 would afford important results We can, notwithstanding, 

 guided by our present experience, employ the measure proposed, 

 as affording without the chance of great error, an easy and 

 beneficial method of comparing the effects of different modes 

 of action in known or proposed machinery. We may assume 

 the following case as determined by practice : that, if the 

 striking surface of the propellers be l(i square feet, and the 

 velocity of their centre of reaction, or of the stream which they 



