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Third Part.



83. In reviewing all the machines intended for raising and removing heavy weights, the most useful and most portable are found to be the screw, the crane, and the tackle.

The screw is more portable than the crane, but is ineffectual in raising heavy blocks; it can only separate them from the ground. The crane, on the contrary, can elevate them to any given height-this depending on the length and force of the rope, chain, or leather strap which rolls upon the windlass or wheel placed at the bottom of the crane, and the number of men who, being furnished with bars, put the machine in action.

The tackle serves the same purpose as the crane, but with more facility and less workmen; this can be adapted to a crane, thus augmenting its force.

An inconvenience of the screw is, its sinking into the ground if it is moist or sandy. A machine would be desirable in which these three might be united.-Such a one M. David How claims to have invented, the advantages of which he thus sums up:

Firstly. This machine is simple and portable.

Secondly. It not only moves the weight, but raises it above the ground.

Thirdly. It so well supplies the power of men, as to cause the strength of four to be more powerful than that of a hundred.

Of this machine, M. How gives the following description:

It is composed of three pieces of wood fifteen feet long, pierced in the top with holes in which a strong iron bar is passed, to which a tackle is hooked. The stake of the hinder one is immovable, and is carefully fixed in the ground; those of the front should have free play; they must also be spread apart, and fixed in the ground.

When the machine is placed, a rope is passed around the wheels of the tackle. A pin of two inches in length-the upper part of which is flat and the lower part cylindrical-bears a ring in which the hook is inserted. This pin is about nine-tenths of an inch in diameter at its lower extremity, and grows somewhat larger towards the middle.

The end of the rope which passes over the pulleys, rolls over a windlass of some six feet or more. A workman then, with a chisel or burin, scoops out a round hole of a little more than an inch in depth, and as nearly perpendicular as possible, in the object to be raised; this hole should be somewhat smaller than the pin, so that it can only be inserted by driving in with a mallet. When the pin is thus forced into the stone, the hook of the tackle is passed in its ring, and the ropes extended by means of the crank of the windlass; all that remains to be done is to place the persons necessary to work it, and, strange as it may seem, the heaviest mass, although only grasped by the pin, can be torn from its bed despite all the obstacles which may oppose it, and raised in such a manner as to be suspended in the air.

To explain this extraordinary fact, those who have had experience think that the pin does not penetrate the stone in the precise direction of the acting force, and that the mass is raised and suspended in the precise direction of the acting force, and that the mass is raised and suspended in the direction pointed by the pins. M. How is not of this opinion; he is confident that it is to the elasticity of the stone, not the direction of the power, that the effects produced are attributable. The pin, being driven by blows of the mallet into the orifice in which it should rest, is retained there by the elastic power of the stone, in the same manner that a similar pin would be fastened in a block of wood if driven in it by the same means; yet with this difference, that the power exercised by the stone upon the iron, is incomparably greater than that exercised by the wood upon it. This explanation has been confirmed by experience, as it has been found: First, that the force which lifts the mass, acts exactly in the axle of the orifice in which the pin is forced; secondly, that when this mass is raised from the ground, it can be made to take every position without detaching itself from the pin; thirdly, that while no effort can draw out the pin, one or two blows of the mallet will detach it with the greatest facility.

The force which retains the iron, varies in proportion to the greater or lesser elasticity of the stone; this force will be less in soft stones than in Marble, granite, porphyry, etc., and it is the opinion of the author that the trial can only be successfully made upon these last.

We can also say that, although one can conceive that a certain point, and in certain cases, large masses of stone can be suspended in the manner we have indicated, it is much more difficult to explain how these masses can be raised in a multitude of inclined and horizontal positions, and how, admitting that it is to the direction in which the pin is enforced that its adherence to the stone is attributable; it is found that, while a constant effort applied in every direction to the stone does not detach it, a slight concussion produces this effect at once.

To prove this, the following experiment will suffice:

Take a pin or iron, force it into a block of granite in the manner which we have described, then, without arranging apparatus, try, by means of a cord attached to the pin, to draw it out in every direction, using the greatest possible force-all your efforts will be useless. This clearly proves that it is the elastic force of the stone, instead of the direction in which the cord is drawn, which retains the pin in its place.

It is even necessary, when apparatus is used, to pierce the stone perpendicularly, because, if pierced obliquely, there is reason to fear that the portion of stone between the iron and the surface will not yield.

It is surprising, in using this machine upon large masses of granite, to see how little hold is necessary to draw them from the ground; sometimes, when the pin is inserted but a third of an inch in the stone, it will be immovable, and capable of sustaining a weight of several thousands of pounds.

This machine, so simple that it can be put up in the quarries and fields as well as in Marble yards, deserves to be practically used. It is necessary to fasten the uprights strongly, and to proportion their strength to the weight to be raised; because if one of them should bend or become displaced, a shock would ensue which would stop the operation, and, probably, cause some accident.

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