at the crank. An endless rope, or one of which the
two ends are spliced together, is passed five or six times round the
small end of the windlass, and down under a single pulley below: then,
as the windlass is turned by the crank, the rope is constantly given
off from one part, while the circumference is greater. Now it is
plain, that if the windlass is one-fifth of an inch larger in
circumference, at the point at which the rope is taken up, than at the
place where it is given off to the pulley, that whatever may be
appended thereto, will be raised one tenth of an inch by each
revolution Then, if we suppose the crank lever to be fifteen inches,
the handle will travel about 100 inches, in each revolution, which
gives a power, or increase of force, of 1000 to one. Therefore, if 100
pounds of power be applied to the crank handle, it will be
sufficient--minus friction--to raise a weight of 100,000 lbs. The only
inconvenience in this apparatus, and which prevents its coming into
more general use, is, that it is too limited in the extent of its
motion, in consequence of the travelling of the rope from one end of
the windlass to the other. Thus, if the windlass be but twenty-five
inches long, and the rope one inch in diameter, it will admit only
twenty revolutions, without renewing. Yet, in many cases, in which an
article in required to be raised, or moved but a few inches, the
conical windlass will be found preferable to any other method.

* * * * *

REQUISITE STRENGTH OF STEAM BOILERS.

Our correspondent S. B. cannot comprehend that the strength of iron
for a cylindrical boiler should Be in direct proportion to the
diameter thereof, in order to sustain an equal pressure per square
inch; wherefore, we must reason with him on the long scale. The
cohesive strength of good iron is 64,000 lbs. per square inch; and of
course, a strip of boiler-iron plate 1/8th inch thick will sustain
8000 lbs. If a boiler made of thin iron is 14 inches in diameter, or
44 inches in circumference, each inch of its length will contain 44
square inches, and either half thereof will contain 22 inches, and as
the pressure on this portion is sustained by at least two inches of
width of plate,--one inch on each side,--it follows that it will
sustain a pressure of at least 700 lbs. per square inch, in the
direction of circumference. If the diameter is double, the number of
square inches will be double, and will re