Fig. 16. Action of the wedge
"The wedge may be a block of wood, iron, or other material, tapered to a thin edge, forming a sort of double inclined plane, A P B, ([Fig. 16]) where their bases are joined, making A B the whole thickness of the wedge at the top. In splitting wood as is shown in the illustration, R R being the wood, the wedge must be driven in with a large hammer or heavy mallet which impels it down and forces the fibres of the wood to separate and open up. The wedge is of great importance in a vast variety of cases where the other mechanical powers are of no avail, and this arises from the momentum of the blow given it; which is greater beyond comparison than the application of any dead weight or pressure employed by the other mechanical powers. Hence, it is used in splitting wood, rocks, and many other things. Even the largest ships may be raised somewhat by driving wedges below them. Often, in launching a vessel, wedges are used to start it on its way. And they are also used for raising beams or floors of houses where they have given way by reason of having too much weight laid upon them. In quarrying large stones, it is customary to wedge or break off the rock by first drilling a number of holes on the line of cleavage. Wooden wedges are then driven tightly into these and left there until they get wet, when they expand and split off the rock as required. This method of quarrying large stones was well known to the old Egyptians, and employed by them in quarrying their famous obelisks.
"Owing to the fact that the power applied to force a wedge is not continuous, but a series of impulses, the theory of the wedge is less exact than that of the other mechanical powers. Considering the power and the resistance on each side, however, as three forces in equilibrium, it may be demonstrated that the
Resistance (R) equals P × Length of equal side/Back of wedge
Then the mechanical advantage will be—
R/P equals Length of equal side/Back of wedge
So that by diminishing the size of the back and increasing the length of the side—that is, diminishing the angle of penetration—the mechanical power of the wedge is increased. While I did not intend to inflict you with arithmetical or algebraical formulæ, I have been compelled to give you that simple example which I know you can all work out, as it is concise, and the same would be long and tedious if rendered in text."
Next morning, as Fred and his father were out on the new place early, looking over the boat house, the slide for the boat, and some other matters, Mr. Gregg suggested that a winch be placed at the upper end of the house, to haul the boat out of the water. He also suggested that Fred prepare for work on the boat at once, and provide himself with all the tools and materials necessary. He promised to call on a friend of his in the city, who is a noted boat builder, and ask him the best method to adopt in building the craft.
"Perhaps," said the father, "it might be a good plan to buy a full set of shapes or patterns from some one of the professional boat builders who advertise such. They are sold at a very low rate—being made of paper—and many firms sell all the material that is required to build a boat complete; with the sweeps, ribs, and curved stuff cut out to the required shape and numbered all ready to set up.
"What we want, Fred," continued the father, "is a boat sixteen or eighteen feet long, just the size of the one belonging to your friend, Walter Scott; that is plenty large enough for all our purposes. His boat can stand as a kind of a model for you to work after in case you do not thoroughly understand the patterns you are to get, or the manner of arrangement. The gasolene motor we'll order from some manufacturer, with whom we'll arrange to install it, with a suitable propeller and necessary attachments."