. 2.

[Illustration: FIG. 2. & FIG. 3.]

By using the third prism in conjunction with the second a still longer
base of one-fourth the distance of the object can be employed. The
range finder can also be used as a depleidoscope for transit
observations. For this purpose it is mounted on a block of wood by
means of elastic band and leveled by the level on its lid, being at
the same time set in the meridian of the place. The lid is opened to
make an angle with the horizon equal to the latitude of the place of
observation. On looking into the upper prism two images of the sun
will be seen on each side of the apex of the prism, which gradually
approach each other as the sun nears the meridian, and finally
coincide as it passes it, the time of which being noted gives the
longitude of the place.

Extensive trials of the instrument have been made both in this country
and in India, which agree in showing that the average error in using
the instrument is about 21/2 to 31/2 per cent.

* * * * *

WHEELS LINKED WITH A BELL CRANK.

[Illustration: FIG. 1]

There are four ways in which a connecting rod is made use of in
machine work. The first is in linking two wheels together that stand
in the same position, but a slight distance off centers. The rod in
this case has only to lead the driven wheel around by connecting it
with the driver, and consequently has only to endure a pulling strain
in the direction of its length. The second is when the rod is called
upon to stand a pull and a push at every revolution. The third takes
in the matter of the twisting strain that a rod can manage; but the
fourth brings the hardest usage that a connecting rod can be called
upon to endure, and that is by making a lever of the rod to get a
driving action by prying on a fulcrum in the center. In Fig. 1 is seen
a case of this kind taken from a machine in which a disk engine was
made use of. The rod has a chance to turn about on its center from a
ball and socket joint, and engages with both wheels in nicely fitted
journals, and boxes set in line with the center of the socket joint,
so that when one wheel turns, the rod pries the other around by using
the rod as a lever and the ball joint for a fulcrum, giving a uniform
leverage all the while, with no dead centers.

[Illustration: FIG. 2.]

To set this arrangement around at right angles, or where the shafts
will bring the wheels together, as for bevel gea