Skip to main content

‹‹‹ prev (8)

(10) next ›››

' campbelt;s registering sux-dial. ^-^ j?qL/p '
^ci/»^ "^ (3W)!,. CAMPBELL'S REGISTERING SUN-DIAl!^^~^--Z^^^*<^^^
AT some time in March or April, 1879,
some gentlemen, and some ladies too,
were very much amazed to find their writing-
arson being suspected, a tumbler was tried
and convicted of the same offence. Dragged
into light he too burned wooS. A knot in
desks and tables smoking. They wrote to ' a pane of glass is a " lens ;" a finger-glass, a
the papers about it. Under the heading, 1 wine bottle, a tumbler, a wine glass, a globe
" An unsuspected danger," a series of letters for gold fish, a chemist's window ornaments,
appeared in the leading journals. Certain I any glass or glass vessel full of clear fluid and
transparent glass balls, now commonly used 1 properly shaped, may happen to be so placed
as paper-weights, being set in sunshine,
" acted like a burning-glass " on tables and
table-covers, to the astonishment of their
owners. They are burning-glasses. In daily
life theory very seldom is jaractised ; know-
ledge very seldom is used. Though "optics''
are taught in schools and colleges, very few
people realise that things polished and
transparent, with surfaces shaped so as to
condense light, are instruments which may
set combustibles on fire.
" Burning-glass '' was defined in the " En-
cyclopaedia Britannica" in 1797, as " A con-
vex glass, commonly spherical, which, being
exposed directly to the sun, collects all the
rays falling thereon into a very small space
called the focus, where wood or any other
as to concentrate sunshine at some hour of the
day, on some day in a year, upon a combustible,
when a breeze of wind may kindle a blaze,
and burn a house or a ship unexpectedly
and unsuspected. Knowing something of
this, about 1853 the writer tried to use some 3,
of the small amount of knowledge which he ''- '"
had picked up. Amongst other contrivances,
he then invented a very simple instrument
founded upon two facts, (t) a transparent
ball is a burning-glass; and (2) the world
turns round. This paper is an attempt to
describe the principle of the contrivance
which a child of six years old understood
some fifty years ago.
A billiard ball is a " sphere." Slices cutoff
it are bounded by circles. All sides of it are
combustible matter being put, will be set on alike. On it, or any other " sphere," great or
fire." In the article quoted Sir David 1 small, may be drawn the lines and scales which
Brewster tells what had been done with
burning-glasses. We have all been taught
that Archimedes burned the Roman fleet at
Syracuse with sunshine, about 2,090 years
ago. The writer's family and Sir David
Brewster were friends, and he had the great
advantage of knowing that distinguished
philosopher, and of learning curious know-
ledge from him in conversation. Some fifty
years ago somebody gave a child an optical
toy, a glass like the bulb and stem of a big
thermometer filled with water. He then
learned experimentally that a transparent
ball is a magnifier, and burns fingers. Ever
since that childish lesson was learned, as boy
and man the writer has been striving to learn
more about " burning-glasses." Neverthe-
less he too was caught unawares by sunshine.
In April, 1879, an innocent egg-shaped
water-bottle burned a hole in a toilet-table,
which happened to be set in a new place
where the sun happened to shine upon it at
the hour when the focus of this burning-
glass happened to be at thfe table on which
the bottle stood. That particular combi-
nation may not happen again for a year, but
every time such a combmation does occur
danger recurs, from the misuse of a bottle
are drawn upon school globes. A trans-
parent sphere placed wliere any sort of light
shines upon it presents the same curves to
the "rays," and bends them to correspond-
ing places on the opposite side, where the
rays cross at a " focus." Whether a sphere
is turned end for end, or capsized, spun, or
rolled, the focus for parallel rays always is
opposite to the source of light, and at a
certain distance from the surface. A spherical
bottle is a lens ; and so is the atmosphere.
When a transparent sphere is set out of
doors all the shining bodies that stud the
visible sky shine through it to opposite foci.
Each forms an image of itsell and a cone of
light, which may be cut by a surface placed
in the cone. The lens of a photographic
camera is founded upon the principle. The
sensitive screen cuts cones of light. The
writer has got images of the stars, the moon,
and the sun, by photography. But the
screen of a camera is flat, not spherical.
Distances measured upon it are unequal, and
drawings are out of perspective, and out of
proportion. The stars and the moon draw,
hut the sun is so hot that the image " burns.''
For many years a lens has been set at Paris
so as to focus the sun's rays upon gunpowder.
The associate of this criminal found guilty of at noon, and so fire a small cannon, Gun

Images and transcriptions on this page, including medium image downloads, may be used under the Creative Commons Attribution 4.0 International Licence unless otherwise stated. Creative Commons Attribution 4.0 International Licence