14 
HISTORY OF THE INVENTION. 
groove, which forces the flat faces in close contact, and forms a very secure 
coupling. 
The shaft Z is carried through a stuffing box fitted into the false stern post, as 
seen in the elevation in the copper-plate, and in the diagram fig. 12. 
The screw is of 5 feet 9 inches diameter, and 8 feet pitch; it consists of two 
threads which are opposite each other, and of equal size and area, so that when 
it is viewed endwise it appears like a complete disc, as represented at fig. 11. 
The length of the screw therefore is four feet. 
Fig. 11. 
The Archimedes was fitted in the first instance with a single threaded screw, 
as shown in fig. 12; but the velocity of the vessel was found not to be so 
Fig. 12. 
great as with the double thread. It also communicated a vibratory movement