1^4 



whilst the oar, re-acting upon the boat, causes it to advance 

 with the difference between the resisting area of the oar and 

 boat. All the sub-aqueous stern propellers — the Archime- 

 dian screw, as it is called, Blaxland's, and others — may also 

 be called rotary sculls, more or less perfect in principle, the 

 great defect of most of them being, that the angle of the 

 blade with the plane of rotation is too small, thereby requir- 

 ing, to produce the required speed in the vessel, far more 

 revolutions in the propeller than the strokes of the engine ; 

 which is accomplished by additional drums and straps, or 

 cog-wheels, by so much increasing the friction, and conse- 

 quent wear and tear of the machinery. 



By the aid of the tin models already referred to, I made 

 the following experiments : — I procured two tin boats about 

 two feet long, which were coupled together, leaving sufficient 

 space between for the propellers to revolve when placed at 

 right angles to the path of the boats. The power applied to 

 put them in motion was a clock spring, whereby each wheel 

 made forty-two revolutions whilst the spring was unwinding. 

 The vanes of the first wheel were at an angle of about 25°, 

 which propelled the boats, by forty- two revolutions of the 

 wheel, 54 links in 35". The angle of the second was 45°, by 

 which, in the same number of revolutions of the wheel, the 

 boats were propelled 120 links in T 15". By the third, with 

 vanes at an angle of 53°, the boats were propelled 150 links 

 in r 43". These several distances coincide with what I have 

 advanced, the distances being in proportion to the tangents 

 of the angles of the vanes ; they show that when the angle 

 of the propeller with the plane of rotation is increased, the 

 resistance to rotation is also increased ; yet that there is no 

 loss of power, but that by so much as the propeller more 

 slowly rotates on its own axis, the boat is by so much more 

 advanced with one revolution of the propeller. I 



By other experiments I also found that there is a certain 



