urd.

The explosive force must be exerted in all directions equally. The real
explanation is, that the explosive action of nitro-glycerine is so
nearly instantaneous, that the resistance of the atmosphere is very
nearly equal to that of the rock; at any rate, is sufficient to cause
the rock to be broken up. The rock yields to the force very nearly as
readily as the atmosphere does.

_Third_. An interesting solution is presented here of what is to many an
astronomical puzzle. When I was younger than I am now, I was greatly
troubled to understand how it could be that if the moon was always
falling to the earth, as the astronomers assured us it was, it should
never reach it, nor have its falling velocity accelerated. In popular
treatises on astronomy, such for example as that of Professor Newcomb,
this is explained by a diagram in which the tangential line is carried
out as in Fig. 1, and by showing that in falling from the point A to the
earth as a center, through distances increasing as the square of the
time, the moon, having the tangential velocity that it has, could never
get nearer to the earth than the circle in which it revolves around it.
This is all very true, and very unsatisfactory. We know that this long
tangential line has nothing to do with the motion of the moon, and while
we are compelled to assent to the demonstration, we want something
better. To my mind the better and more satisfactory explanation is found
in the fact that the moon is forever commencing to fall, and is
continually beginning to fall in a new direction. A revolving body, as
we have seen, never gets past that point, which is entirely beyond our
sight and our comprehension, of beginning to fall, before the direction
of its fall is changed. So, under the attraction of the earth, the moon
is forever leaving a new tangential direction of motion at the same
rate, without acceleration.

(_To be continued_.)

* * * * *

COMPRESSED AIR POWER SCHEMES.

By J. STURGEON, Engineer of the Birmingham Compressed Air Power Company.

In the article on "Gas, Air, and Water Power" in the _Journal_ for Dec.
8 last, you state that you await with some curiosity my reply to certain
points in reference to the compressed air power schemes alluded to in
that article. I now, therefore, take the liberty of submitting to you
the arguments on my side of the question (which are substantially the
same as those I am submitting