100 Answer to the Geological Queries of a Constant Reader. 
pelled from the circumference of the axle which is surrounded 
by it in the box. Any requisite supplies of water or of oil can 
be furnished to the box by a variety of simple means ; and thus 
the axle may be allowed to revolve in circles, whose concave 
perimeters will not even embrace it water-tight, and yet pre- 
serve the air securely from escaping. 
The centre of a vessel is that part which is least affected by 
any agitation of the elements ; and it may, perhaps, be practica- 
bie to assign a central position to the wheel; and if that assign- 
ment of place be practicable, the wheel may be made to act effi- 
ciently when the whole circumference revolves at any depth be- 
low the external surface of the water through which the vessel 
moyes. An inclosed wheel may be comparatively small in dia- 
meter, because the descending planes strike the water horizon- 
tally without any obstruction to their progress; a waste of 
power will be saved, because the resistance to motion will be the 
pressure of water upon the area only of a single plane; and 
the exterior case will be an important protection to the wheels. 
As an act of amusement, a strong hox had its top and both 
ends removed, and was fastened by iron bars to the stern of a 
heavy row-boat at sea; a wheel was placed upon the sides of 
the box with its axle touching the water, and then made to re- 
Volve by pressing with the right and left hand alternately upon 
the perimeter. The boat was impelled half a mile through the 
waves ; but the power was badly placed, and likewise too small 
to give any velocity to the wheel. The stern is the very worst 
position for a wheel; the water at the stern of the boat is 
lowered by the moving planes, and much efficient pressure is 
destroyed by the act. 
E. 
XXI. Answer to the Geological Queries of a Constant Reader®. 
To Mr. Tilloch. 
Siz, — Lo the questions put to me by a correspondent, at 
p- 12 of your last Magazine, on the subject of the different ré- 
positories of coal in the NE part of England, I shall reply with 
as much brevity as the subject will admit, and hope my answers 
may be found satisfactory. 
To query 1.—Mines of coal and quarries of encrinal limestone 
are worked at short intervals from the sea-coast of Northumber- 
land N of the Coquet in a direct line to Alston Moor; but the 
* See our last number, p. 12, 
valuahle 
