I Wish that every one of my boy readers could enjoy himself for a day in that marvelous place, the South Kensington Museum, where are exhibited all kinds of machines costing hundreds of thousands of dollars, which any visitor may work by merely pushing a button. There are so many things of interest that one would hardly know where to begin to describe them, and there are thousands which of course boys could not attempt to make.

In one of the upper galleries of the museum I once came across a room filled with cases containing all the principal mechanical movements known to engineers. Although you may not care to make these I describe, I have no doubt you will agree that they are interesting for their ingenuity and what they do.

The first one is shown in Figures I and 2. I will merely explain its working, and you can see from the drawings how to make it, using cigar-box wood for the parts.

When the wheel T is turned by the crank behind, - the bearing for the wheel being in the back board A,- the pins or nails N in its rim catch upon the horizontal part of the bell-crank L. This shoves the square rod R to the right as the wheel turns, as you can see.

When the lever I is moved down half of the distance between two of the nails on T, the nail runs over its end, as in the drawing, Figure 2, where the nail is shown just leaving. At the same time the next nail in advance of this one hits a small block b on the right and shoves it back where it started from, or into the position shown by the dotted line.

This nail N then slides off the top of the block b, and just as it does so the lever I has caught up another nail and shoves it back to the right, so that as long as T rotates, the rod R will be shoved back and forth, one time for each nail in the wheel T.

The guides for the rod and the patterns for the lever I are shown in separate drawings, and by making your model as shown and being careful about spacing the nails equal distances apart, you will have no trouble in making a model like this for yourself.

If this is too hard, Figure 3 shows a simpler way of accomplishing the same thing. In this the rod or frame A moves back and forth only three times for one rotation of the wheel T.

In this case T is a triangular wheel, each side being a curve, - the part of a circle drawn from the point of the opposite side.

This is explained in the smaller drawing, Figure 3, where A is shown as the center of the curve b-c which is drawn first. Leave your compasses set as they are, and, taking the point b on b-c, draw the curve a-c with b as the center. Where these two lines cross, you have the point c. Taking this as a center and with the same radius, draw a-b.

Thus, b is the center for c-a, c is the center for b-a, and a is the center for b-c.

The axle A is placed in the center of this wheel, which center is found by running lines across from each corner to the center of the opposite sides, one of these lines being shown dotted. Where these three lines cross is the center of the wheel.

Now saw a rectangular hole in a piece of board, as high up and down as the dotted line on the triangle we have just cut, plus one-eighth of an inch, and somewhat wider than it is high. The shape of this hole S is clearly shown in the drawing.

You can saw the outside into a circle as I have shown or leave it in any other shape, but from each side of it run projections, A, forming the rod which is to vibrate back and forth.

These are held in place by guides G, as you can see.

The axle for T runs through the back board on which the guides G are nailed.

Now, when you turn T, each point, as it touches a side will push that side from it.

The point above, in the position shown, has pushed the piece A up, but when the wheel turns a little more to the right, one of the points below will begin to push down again, and so it works three times for each turn of T.

The third mechanical movement consists, as in Figure 4, of a crank C, the crank pin being shown at K and the axle at A.

When K revolves in a circle, K slides in the slots S in one arm of the "bell cranks" L, which are on screws at b. Thus the crank shoves these levers to the right and left, which in turn works the horizontal ends of the "bell crank" up and down; when the upper one is going up, the lower one is dropping and vice versa.

This principle is used in a double acting pump in which one piston goes up while the other goes down.