ON THE POSITION OF THE SCREW. 
41 
ascertained. If it were not so, the method of estimating the area would be 
simple enough, being founded on the same principle as we have adopted in 
ascertaining the effective resistance when compared to the power consumed. 
For as this resistance must be equal to a flat surface moving in the line of 
the vessel’s motion, and the power consumed equal to the sum of the effective 
and ineffective resistances, the latter will represent the area of the screw, and 
the former that of a flat surface; so that by referring to the Tables at page 32, 
we conclude that the area of the screw (figs. 25 and 26), in order to produce a 
propelling resistance equal to a flat surface, should be as 
488 : 1000, or nearly 2:1. 
And to find the area of such a screw to produce a propelling effect equal to that of 
a paddle wheel, the floats of which are of known area, 
By referring to page 29, we find the effect of a paddle compared to the power 
consumed, that is to say, to the action of a flat surface moving at a right angle 
to the keel, is as 
14 : 10 = nearly about j^ths of a flat surface. 
Hence the area of the screw to produce a resistance equal to that of the floats of 
a paddle wheel would be in the proportion of 7 : 5. 
It need hardly be stated that this would be in a great degree dependent on the 
intervals between which the water is acted upon in both cases; that is to say, 
by the number of floats in the paddle wheel, and the number of threads in the 
screw; for unless an equal quantity of water is put in motion in each instance 
the areas must necessarily vary from the preceding proportions. In the paddle 
wheel the number of floats which may be advantageously used is pretty well 
ascertained, but it is experiment only which can decide the maximum surface 
as well as the number of the threads, the latter of which form a number of 
successive points at which the water receives the impulse of the screw. A sensible 
improvement was produced in the speed of the Archimedes by substituting the 
double thread for the original single one. 
ON THE POSITION OF THE SCREW. 
We have next to consider whether the situation in the dead wood is the best 
that could be selected. If there be but one screw, the only position in which it 
could be fixed to give direct motion to the vessel is at the stern or bow. If at 
the stern, then the dead wood is the only place in which it could be placed 
without interfering with the rudder. But it might be fixed in the bow, as in the 
arrangement of Mr. Brown, page 7; and it is by no means certain that this 
disposition would not in still water be equally effective with the arrangement 
G