108 Notices of the Proceedings of 



create be 10 nautical miles per hour, this will produce a ve- 

 locity of 7 miles per hour, in vessels of an advantageous shape^ 

 measuring about 300 Inns. In all cases where a stream of ilitf 

 same section and velocity, or of ihe same dynamic effect when 

 these vary, can be produced, we may decide that the effect 

 ■will be the same. Representing the moving power in this 

 case by the number ifiOO, we may estimate the effect of any 

 machinery as of the same amount, when the section of its 

 stream multiplied by the square of the effective velocity of the 

 striking part, produces this number. 



In determining the value of different kinds of propelling 

 machinery, we have only to ascertain the greater or less amount 

 of power in the internal machinery, which will be required, 

 when the contiivance works under such conditions as to pro- 

 duce the result above-mentioned. • 



All kinds of propelling machinery may be divided into two 

 classes, according as the movement is rotatory or rectilineal 

 and each division is susceptible of two subdivisions, according 

 as the striking surface meets the water directly Or obliquely. 

 There are therefore lour varieties : 



1. The fust, viz. when the movement is rotatory and the 

 pressure direct includes the oar and the common paddle wheel. 

 The latter is in principle the same as the former, deprived of its 

 back stroke, or reciprocating movement. So great are the ad- 

 vantages of the common paddle wheel, on account of its slight 

 friction, its simplicity, and its regularity of movement" that it 

 will probably never be superseded in its application to ordinary 

 purposes. The simplest form of it has hitherto retained its 

 super-iority over the complex contrivances proposed for render- 

 ing its action more advantageous. • In fighting ships or bat- 

 teries moved by steam, it has occasionally been placed in- 

 ternally, as affording greater security to it. It might in such 

 a case be secured beneath the surface of the sea, by being 

 placed horizontally: the water being supplied from forwards, 

 jso as to be led in at the centre of the wheel ; but propelled 

 "backwards from its circumference, #s happens to the air in 

 certain blowing machines. The power required to produce a 

 given effect, would in this case be much increased. 



2. The second variety, which is when the movement is ro- 

 tatory and the impact oblique, offers the construction proposed 

 in the first memoir by Captain Pole, and by many others, in 

 several different forms. A patent seems, to have been got for 

 it, about thirty years ago. It may be described as identical 

 with the arms and vanes of a windmill or smoke jack, revolv- 

 ing under water. On account of the obliquity of its surface, the 

 vane must move about twice as fast as the stream it produces. 



