27 
peller blade as used in the navy, that is, the quantity rcosv 
upon cosa is a constant quantity for every point on the 
screw blade, viz., the pitch of the screw. 
It is no part of my design here to investigate the law 
which connects the distance {x) of the screw from the stern 
of the vessel with the velocity V of the water in which the 
screw revolves, the admission of a quantity Y is sufficient 
for my present purpose. 
It may, however, be observed that the velocity V varies 
from the velocity (v) of the ship at the stern, to the velocity 
zero at a point which is the limit of the wake of the ship. 
Those who are interested in and investigating this subject 
would do well to consult two papers printed in the Transac- 
tions of the Society of Naval Architects, by Professor 
Osborne Eeynolds. The first paper is entitled “On the 
Effect of Immersion of Screw Propellers,” (see Transactions 
of Ins. N. Architects, 1874*, page 188). The second paper is 
entitled “ On the Unequal Onward Motion in the Upper and 
Lower Currents in the Wake of a Ship, &c,, (see Tran, of I. 
N. Architects, 1876). 
18. The most recent experience in screw propulsion is 
given in a paper by James Wright, Esq., Vice-President of 
the Institution of Naval Architects, &c. The title of this 
paper is “ The Steam Trials of Her Majesty’s Ship Iris it 
was read and discussed at the Institution of Naval Architects, 
April, 1879. 
Here, in a four-bladed common screw, a negative slip is 
recorded, which increases from 1’57 to 5 ’3 3 per cent, while 
the speed diminishes from 16 to 8 knots per hour, and the 
I.H.P. diminishes from 7,503 to 755, and the revolutions 
per minute diminishes from 91 to 43. This circumstance 
is passed over in silence by Mr. Wright, who very justly 
directs the attention to the fact that a two-bladed screw is 
a more effective propeller than a four-bladed one of the 
same dimensions. 
