Encyclopaedia Britannica, or, a Dictionary of arts, sciences, and miscellaneous literature : enlarged and improved. Illustrated with nearly six hundred engravings > Volume 18, RHI-SCR

AMUSEMENTS OF SCIENCE.
*54
Fig. 60.
ihecliamcal advantage. By making this wedge in two
parts hinged together, with a graduated piece to keep
them afunder, the wedge may be adjutted to any given ob¬
liquity j and it will always be found, that the mechanical
advantage of the wedge may be afcertained by compa¬
ring its perpendicular elevation with its bafe. If the
bafe of the. wedge be 2, 3, 4, 5, or any other number of
times greater than its height, it will enable the perfon
■to raife refpeftively 2, 3, 4, or 5 times more weight than
he could do in experiment 1. by which his power is efti-
mated.
13. The Screw.
The fcrew is an inclined plane wound round a cylin¬
der : the height of all its revolutions round the cylinder
taken together, compared with the fpace through which
the power that it turns paffes, is the meafure of its me¬
chanical advantage. Let the lever ufed in the laft expe¬
riment be turned in fuch a manner as to reach from its
gudgeon to the draft of the Panorganon, guided by an
attendant lever as before (fig. 60.). Let the wheel
reft upon the loweft helix or thread of the fcrew ; as the
arms of the draft are turned round, the wheel will
afcend, and carry up the weight which is fattened to the
lever. As the fituation df the fcrew prevents the rveight
from being fufpended exadlly from the centre of the
fcrew, proper allowance muff be made for this in efti-
mating the force of the fcrew, or determining the me¬
chanical advantage gained by the lever. This can be
done by meafuring the perpendicular afcent of the
weight, which in all cafes is ufeful, and more expedi¬
tious than meafuring the parts of a machine, and efti-
-mating its force by calculation ; becaufe the different
diameters of ropes, and other fmall circumftances, are
frequently miftaken in eftimates—both methods diuuld
be employed and their refults compared. The fpace
paffed through by the moving power, and by that which
it moves, are infallible data for eftimating the powers of
engines.
Two very material fubje&s of experiment yet remain
for the Panorganon j fridion, and wheels of carriages j
but perhaps we may be thought to have extended this
feftion beyond its juft proportion to the reft of the ar¬
ticle, in which it is not intended to write a treatife upon
fcience, but to point out methods of initiating young
people in the rudiments of knowledge, and of giving
them a diftinft view of thofe principles on which they
are founded.^No preceptor who has had experience will
cavil at the fuperficial knowledge of a boy of 12 or 13
upon thefe fubjefts; he will perceive that the general
view which we with to give, mutt tend to form a tafte
for literature and inveftigation. The fciolijl has learned
only to talk—we wilh to teach our pupils to think up¬
on the various objefls connefted with the prefent ar¬
ticle.
The Panorganon may be employed in afcertaining
the refiftance of air and water; the force of different
mufcles} and in a great variety of amufing and ufeful
experiments. In academies and private families, it may
■3 Eige- be eredted in the place allotted for amufement, where it
ivarth s furnilh entertainment for many a vacant hour.
^Education When it has loft its novelty, the fhaft may from time to
vol. ii. vhap. time be taken down, and a fwing may be fufpended in
zvih its .place *e
Sect. XII. Recreations and Contrivances relating to R^r”1
Optics. tiom'"
In the articles Catoptrics, Dioptrics, Micro- ss k
SCOPE and PERSPECTIVE, we have defcribed a variety Optical res:
of optical recreations, viz. under Catoptrics, Seft. 11 creations,
CATOPTRICAL ILLUSIONS •, the appearance of a bomdlejs
vijla; a fortification apparently of immenfe extent j a
furprifing multiplication of objedbs •, the optical para-,
dox, by which opaque bodies are feemingly rendered
tranfparent •, the magician's mirror ; theperfpedlive mir¬
ror ; the adtion of concave mirrors in inflaming com-
buttible bodies, and the real apparition. Under Diop¬
trics, page 244 of Vol. VII. opticalillufions ; the opti¬
cal augmentation, opticalfubtraBion ; the alternate il/u-
Jion ; the dioptrical paradox ; the camera obfcura ; the
method of (hewing the fpots on the fun’s dilk, and mag¬
nifying fmall objedls by means of the fun’s rays ; the dia-
■gonal opera glafs; the conftrudlion and ufes of the ma¬
gic lantern ; the nebulous magic lantern ; method of pro¬
ducing the appearance of & phantom on a pedeftal placed
on the middle of a table4 and the magic theatre. Un¬
der Microscope, befides fully explaining the conftrue-
tion of the feveral kinds of microfcopes, and explaining
their ufes, we have given an account oPa great variety
of objedls which are feen diftindtly only by means of
thefe inftruments j fuch as the microfcopic animalcu/a ;
the minute parts of infeBs ; the ftrudlure of vegetables,
&c. ; and under Perspective, we have defcribed and
explained the anamorphojis, an inftrument for drawing
in perfpedtive mechanically, and the camera lucida of Dr
Wollafton. Under Optics, Part III. Chap. I. we
have explained the conftrudlion of the principal optical
inftruments, as multiplying glajfes, mirrors, improve¬
ments on the camera obfcura, by Dr Brewfter and Mr
Thomfon *, microfcopes, telefcopes, and various kinds of
apparatus for meafuring the intenfity of light. Under
PyROTECHNY, N° 150, we have fliown how artificial
fireworks may be imitated by certain optical deceptions.
At prefcnt we (hall only defcribe one or two addi¬
tional optical recreations, and explain the nature of the
optical deception called Phantafmagoria.
Experiment to Jhow the Blue Colour of Shadows formed
in Day-Light.
Darken a room in daylight, or towards twilight, fo
that only a fmall proportion of light may enter by the
(butter. Then holding a lighted candle near the open¬
ing of the (butter, caft the (hadow of an objedl, fuch as
a fmall ruler, on a white paper. There will in general
be feen two (hadowTs, the one blue, and the other orange >
the former of which refembles the blue colour of the
(ky in clear funthine, and is of a greater or lefs intenfity
according as the objedl is brought nearer to a focus.
For explanations of the blue colour of the Iky, fee
Optics, Part II. Sedl. 4.
The Air-drawn Dagger.
if
An improved variety of the experiment defcribed un-The air-
der Catoptrics, N° 14. by the name of the realappa1-drawn dag-
rition, is thus defcribed by Montucla. Fig. 62. rePre‘gj,
fents a different pofition of the mirror and partition
from that defcribed under Catoptrics, and one better
adapted