J. F. Campbell Collection > Campbell's registering sun-dial

GOOD WORDS.
powder explodes at 700", which is hot. But
that sort of lens must be turned towards the
source of light so as to get a focus. Tele
scopes and microscopes, and such like, are
made with very small arcs of spherical sur-
faces, which must be aimed at the object
whose rays are to be focussed. .After some
thought and sundry failures, a trans|)arent
globe was seen to be the thing needed for
the purpose wanted. But in 1S53 no glass
globes were to be got in Paris. Hollow
glass globes, blown for lampshades, were to
be had in abundance. These spherical
bottles were then commonly useil in Parisian
workshops to concentrate lamp light upon
work, and to save working eyes iVom dazzling
and from darkness. Through life the writer
has been given to expedients. When he
could not get what he wanted, he did the
best he could without it. He wanted a
spherical lens, and could not get anything
better than a bottle-stopper made of solid
glass. So he found out a maker of lamp-
shades and fraternized with him in his work-
shop, where he lived with a wife, and a
tame squirrel, and a canary, and some
flowers, at the top of a high house where the
sun shone cheerilv. A capital water-lens
was got for a franc. The diameter was
si.x inches, the radius of the sjjhere three,
and a cone of sunlight stretched three inches
Irom the glass to the place where the sun's
image burned. " My faith ! " said the French-
man, when he saw what his glass bottle
could do in sunshine, "you do not sleep all
night long." A London turner was set to
make a bowl of hard wood ; — a hollow half-
sphere, with a radius of six inches, and a
diameter of twelve. The lamp-glass was set
upon a tumbler three inches high, in the
bowl of wood, in a window facing the south,
and this " sun-dial," in various shapes, has
gone on working for the inventor ever since.
.•Vnybody can make it, or use it ; anybody
may, it is not a " patent." One added to
one makes two. One fact added to another
makes an " invention." It does not make
the matter clearer to express it by figures,
1 + 1 = 2, or by .Algebra.
This contrivance is an astronomical engine,
and the first of its kind. A graving tool is
set to draw circles upon a sphere. It is a
"pencil of rays" as thick as the diameter of
the transparent lens, with a conical point
about half a radius long, when the globe is
of solid glass. The centre of the globe an-
swers to the " rest " of a turning lathe ; the
earth's rotation is the machinery in motion.
The long end of the lever reaches from the
sun to the centre of the lens ; the short end is
the hot cone, which burns wood like iron
lieaied to 700^. The invention is a simple
application of natural force and movement.
Like the application of steam or water-power
or the wind to move engines, there is nothing
new in the invention, except the new com-
bination of a transparent sphere, with a
spherical surface so fitted that one concen-
trates a cone of sunshine which the other cuts
at right angles, where it is hottest, whatever
may be the sun's altitude or declination while
it shines upon the glass sphere. All other
dials work on the same principle : this one
uses light instead of shade, ami registers
natural phenomena. As the world turns, the
dial turns with it Eastwards. The hot sun
appears to travel from east to west in the sky,
the hot image of it travels from west to east,
and engraves the path which the sun de-
scribes, measure for measure and rate for
rate. If a cloud stops sunshine during four
minutes of time, the circle drawn with the hot
pencil is broken for a space of one degree on
the circle. No matter what the radius may
be, the angular space is measured by the
world's movement. If the sun shines for an
hour, the arc drawn is fifteen degrees, mea-
sured astronomically for time and for angular
distance. If sunlight is hindered, the hot
image is not so hot ; the point of the pencil
is shortened, the power to sink into wood is
less, and the mark engraved is shallower. It
has been found experimentally that the power
is greater, and the mark deeper, the nearer
the sun is to the zenith. The nearer the sun
is to the horizon, the shallower is the mark.
Apparently the reasons are that the lower air
contains more matters which stop light, be-
cause there is more of the atmosphere in the
way at sunrise and sunset and in winter, and
because spaces between clouds are narrower
when light strikes through layers horizon-
tally, instead of vertically ; level, instead of
downwards. As the world goes round the
sun, lines described on the dial are those
which are expressed on some school globes
between the tropics, and are measured upon
the " Ecliptic." But these being drawn
astronomically, are accurately engraved. If
only the surfaces are made true, the en-
graving must be more accurate than anything
drawn by hands, or by clockwork, or by
dividing engines. No matter how minute
the scale may be, the work done must be
done exactly ; because the world moves, the
dial and sunlight engraves it.
I can think of nothing better for the pur-
pose of registering sunshine than well-made