21 
Ordinary Meeting, November 4th, 1879. 
J. P. Joule, D.C.L., LL.D., F.KS., &c., President, in the 
Chair. 
Screw Propulsion,” Part II., by Egbert Eawson, Assoc. 
I.N.A., Hon. Member of the Manchester Literary and Philo- 
sophical Society, Member of the Mathematical Society. 
7. The articles in Part I. have been principally devoted 
to the consideration of the normal velocity of the propeller 
blade, an element which chiefly, if not entirely, regulates 
the thrust of the screw in the direction of its axis; but 
recent experience in screw propulsion seems to point to 
another resistance arising from the friction between the 
water and the surface of the propeller blades usually called 
shin resistance. This skin resistance, therefore, appears to 
depend, in some way or other, upon the velocity of the 
rubbing surfaces. (See a paper by Eankine on the Mechani- 
cal Principles, of the Action of Propellers.— Transactions of 
the Institution of Naval Architects, 1865, page 21.) 
Hence, the necessity of determining the velocity of the 
element (a) in a plane perpendicular to its normal line. 
8. This may be done as follows : 
Let HP, in the annexed 
figure, be perpendicular to 
the plane AGP. Now the 
velocity (rw) acts in the 
direction of HP, resolve this 
velocity perpendicular to the 
normal PI, it will become 
ru’sinv in the direction HI 
and in the plane HPI, which 
is evidently perpendicular to 
the plane AGP. Again, re- 
solve the velocity (v), which acts in the direction PK, 
PROCEEDiNas — L it, & Phil, Soc, — Vol.XIX. — N o. 3. — Session 1879-80. 
