HISTORY OF THE INVENTION. 
17 
D 
unshipping the screw in the event of its becoming injured by striking any object, 
or by shot or any other cause : in any of these cases, as it is wholly immersed, 
repair would be impracticable unless it could be unshipped. The arrangement in 
the Archimedes to effect this object is shown at fig. 16. a a is an end view 
Fig. 16. 
of the screw in its place in the dead wood. Immediately over the screw there 
is a rectangular trunk c carried up to the deck, through which there is a free 
communication large enough to allow a man to get access to the screw. When 
it is desired to remove the screw, a line is passed down through this opening, 
to which a small buoy is attached. This buoy, being once clear of the side of the 
aperture, rises through the water and carries the line along with it. The buoy 
is then hooked up, and the end of a chain or rope b is attached to the line. By 
hauling the line through the trunk c, and slacking out the chain or rope at the 
same time, the end is brought into the aperture so as to be laid hold of. It is 
then hooked into a hole in the screw, made for that purpose. Two similar 
tackles, previously attached to the transverse spar above the deck, are at the 
same time affixed to the screw axis before and behind the screw. The long 
bolts which extend from the deck to the bearings are then withdrawn, and the 
bearings are consequently disengaged from their seats. By now pulling a little 
at the tackles attached to the axis the screw becomes suspended, and by hauling 
up the blocks connected with the chain or rope b it is moved out of the perpen 
dicular, so that by gradually slackening the tackles last named, and hauling b at 
the same time, the screw is hoisted close up to the derrick e, and thence easily 
placed on the deck.