208 Natural and Artificial Flight. (April, 
them reciprocate, the one screw producing the current on 
which the other rises, as happens in natural wings.” 
“The Aérial Wave Screw operates also upon Water.—The form 
of screw just described is adapted in a marked manner for 
water, if the blades be made of carefully tempered finely 
graduated steel plates and reduced in size. It bears the 
same relation to, and produces the same results upon, water 
as the tail and fin of the fish. It throws its blades during 
its action into double figure-of-8 curves, similar in all respects 
to those produced on the anterior and posterior margins of 
the natural and artificial flying wing. As the speed attained 
by the several portions of each blade varies, so the angle at 
which each part of the blade strikes varies; the angles 
being always greatest towards the root of the blade, and least 
towards the tip. The angles made by the different portions 
of the blade are diminished in proportion as the speed with 
which the screw is driven is increased. The screw in this 
manner is self-adjusting, and extracts a large percentage of 
propelling power with very little force and surprisingly little 
slip. A similar result is obtained if two finely graduated 
angular-shaped steel plates be placed end to end (vertically 
or horizontally matters little), and applied to the water by a 
slight sculling figure-of-8 motion, analogous to that executed 
by the tail of the fish, porpoise, or whale. If the thick 
margins of the plates be directed forwards, and the thin ones 
backwards, an unusually effective propeller is produced. 
This form of propeller is likewise very effective in air.” 
IV. THE GEOLOGY OF THE STRAITS OF DOVER 
By WILLIAM ToPLEY, F.G.S., 
Geological Survey of England and Wales. 
ey : 
NGINEERS have for long busied themselves with 
aH, discussions upon the feasibility of connecting Eng- 
land and France by railway, and during the last few 
years the question has come much before the general public. 
The late war for a time laid all such discussions on one side, 
but recently they have been revived, and once more have 
come prominently forward. 
* The authorities especially referred to in fhe preparation of this paper are 
as follows:—the Admiralty Charts; the Maps of the Geological Survey of 
England, Sheets 3 and 4; the Memoir on Sheet 4, by Mr. F. Drew; the 
Papers by Mr. PuiLuips and Dr. Firron, in the ‘ Geological Transactions,” 
