AREA OF THE SCREW. 
39 
line ec, and by supposing the same to be generated round a cylinder equal to 
the proposed diameter of the screw, we find the angle of the thread. 
Or it may be ascertained by calculation, thus: 
Divide the velocity of the vessel in feet per minute, + the assumed amount 
of slip, by the number of revolutions the screw is intended to make per minute, 
and the quotient will be the pitch of the screw in feet. 
Example.—It is intended that the speed of a vessel shall be 10 miles per hour, 
and the slip is expected to be equal to 2 miles^per hour; required the pitch of 
the screw making 120 revolutions per minute? 
feet. 
Velocity of the vessel in feet per minute 860 
Slip of the screw one-fifth of the above 172 
Then say 
revolutions, feet, revolution. 
1032 
120 : 1032 :: 1 Q . c , , f 
= 8’6 feet pitch of screw. 
TO FIND THE PROPER AREA OF THE SCREW. 
The data by which this may be accurately ascertained cannot be furnished until 
our experience is more matured by practice ; for until then we cannot determine to 
what extent the thread should be carried so as to impart all the motion to the fluid 
which may be advantageously communicated without having unnecessary surface. 
It would be easy to submit rules which look well on paper, but much harm is 
done by going a single step further than we can advance on sure grounds. The 
area of the screw therefore cannot be determined from our present experience, 
but its diameter may be nearly so, for as the ultimate object is to give motion 
to a cylinder of water of equal area to that of the screw, whether or not it may be 
ultimately found that its construction should be the same as that of the Archimedes, 
(that is to say, that it should fill the entire cylinder,) the diameter may be nearly 
determined. We say nearly, because there is a curious circumstance to which 
we must advert, which may probably affect this part of the calculation. General 
Alexander Sabloukoff, of the Russian service, having applied the screw in several 
instances in his own country as a ventilator and blowing machine 11 with complete 
success, and in one instance also as a propeller, communicated the result of his 
11 The screw has recently been used (and we believe patented) as a blowing machine in this country. 
The prior claim to the merit of its application is however due to General Sabloukoff, who circulated 
several copies of the Memoir, referred to in the note on the next page, in this country in the early part 
of the preceding year (1841).