Crawford avalanche. (Grayling, Mich.) 1879-19??, 1893-02-16

TREES STRUCK BY LIGHTNING. Mr.. ID.
| Joncsco has compiled statistics concern
| ing trees which are struck by lightning,
aud arrives at the following interesting
conclusions, which are given In the last
issue OF the ElectricR World: II has
been supposed for long time that Ger
tain kinds Of trees are particularly sub
ject to destruction by lightning, while
others are quite free from danger. Among
the first Is the oak and among the latter
the laurel. An investigation Of this state
ment led the writer 10 the following con-
clusions; cat very high potential cleo
trical discharges all kinds of trees may
be subject to destruction from lightning
Trees containing oils are less subject to
be struck; those containing very much
oil being protected the most, Light
ming appears to prefer those trees which
contain oil to slight degree only in
summer. The quantity OF water con-
tanned in the trees has no effect on their
liability to being struck. Dead limbs of
trees, OF those containing starch as well
as of those containing oil, are particularly
subject to being struck. Cambium, bark
and leaves do not alter the conductivity
of trees. The nature of the soil has no
connection with the frequency of the de-
struction of the trees by lightning"
FOUNDATIONS FOR SEA WALLs.-Suc-
cessful methods in securing suitable
foundations for sea walls are among the
most important of modern engineering
triumphs, and one of the latest of these
is what has come to be known as the
Shield system. originated abroad, and of
special adaptation where the bottom is
of irregular rocky service. Careful longi-
tudinal sections are taken along the
lines of the inner and outer faces of the
foundations, and planks cut and nailed
to timbers in such manner that one
edge of the finished frame conforms ex-
actly with the rocky bottom; these
frames are weighted and sunk into
place, where they are fastened by
long bolts wedged into the rock
in such manner that mould for about
twenty feet of the bottom of the wall is
thus formed. The tops of the frames
are brought to true horizontal position
by means of spirit level, and the space
between is filed with concrete; where a
large mass of the latter is used, it is COM
posed of four to four and one-half parts
of sand and gravel to one of Portland ce
ment, the top being finished off with
concrete composed of one and half to
two parts of sand and gravel to one of
cement. The material is lowered in
canvas bag holding about two and one
fourth cubic feet, and so arranged that
the bottom can be easily opened when it
touches the spot where the concrete is to
be laid. The material as it leaves the
bag is simply pressed down by the flat
hand to the required level, and then gen
tly struck off by straight edge resting
at each edge on the frames. In this way
perfect surface is formed five or six
feet below water, becoming hard enough
in three days to receive the blocks Of the
wall.
VELOCITY OF RAIN DRoPs.-Of course
we all know that it would be an utter
impossibility for storm clouds to form
and rain to fall were it not for the forty
odd miles Of atmosphere that rises above
our heads; but, supposing it were posi-
BILE for human beings to exist in an at
mosphere that only rose to a level with
their mouths, and that storm clouds
could form in the region outside such q
low-grade atmosphere, then every rain
drop would prove as fatal to earthly
creatures as If it were a steel bullet fired
from a dynamite gun. All falling bodies,
whether they be crystal rain drops or
meteortes. fall with what philosophers
term ga uniform accelerated notion" in
other words, if body be moving at q
certain velocity at the expiration of one
second from the beginning of its fall, it
will be moving with twice that velocity
at the expiration of two seconds, gaining
in speed at a uniform rate throughout
the course of its fall. Careful expert
ments have shown that the rate per sec
ond at which bodies acquire velocity in
falling through the air is thirty-two feet
per second at the end of the first second
from starting. At the end of the next
second it is going at the rate of sixty
four feet per second, and so on through
the whole time of falling. Where the
velocity is known the space through
which the body has fallen may be
ascertained by multiplying the velocity
at that period by the number of seconds
during which it has been falling, and
dividing the result by two. This rule
applies, however, only to bodies falling
through a vacuum. The resistance of
our atmosphere materially retards rain
drops, hailstones, acrolites and all other
bodies which fall through it, and were it
not for the resistance it presents, every
rain storm would be disastrous to the
human race, as each drop would fall with
a velocity great enough to penetrate the
full length Of full-grown mans body.
Il Nature.