SHA 
men* and pistils ; these having been obsetV'* 
ed with more accuracy since the discovery 
of the uses for which nature has assigned 
them, a new set of principles have been de- 
rived from them, by means of which the dis- 
tribution of plants has been brought to a 
greater precision, and rendered more con- 
formable to true philosophy, in this system, 
than in any one of those which preceded it. 
The author dues not pretend to call it a na- 
tural system, he gives it as artificial only, 
and modestly owns his inability to detect 
the order pursued by nature in her vegetable 
productions ; but of this he seems confi- 
dent, that no natural order can ever be 
framed without taking in the materials out 
of which he has raised his own ; and urges 
the necessity of admitting artificial systems 
for convenience, till one truly natural shall 
appear. Linnaeus has given us his “ Frag- 
menti Method! Naturalis,”in which he has 
made a distribution of plants under various 
orders, putting together in each such as ap- 
pear to have a natural affinity to each 
other ; this, after a long and fruitless search 
after the natural method, he gives as the re- 
sultof his own speculation, for the assistance 
of such as may engage in the same pursuit 
hereafter. Not finding it practicable to 
form a system after the natural method, 
Linnaeus was more fully convinced of the 
absolute necessity of adopting an artificial 
one, of which a detailed account is given 
under the article Botany. 
SHAD, in ichtliyology, a species of 
Clupea, with the upper jaw bifid at the ex- 
tremity, and spotted with black ; it greatly 
resembles the common herring, and is, on 
that account, sometimes called the mother of 
herring ; all tlie fins are whitish, except that 
on the back ; the tail is very much forked. 
SHADOW, in optics, a privation or di- 
minution of light, by the interposition of an 
opaque body ; or it is a plane where the 
light is either altogether obstructed, or 
greatly weakened, by the interposition of 
some opaque body between it and the lumi- 
nary. A shadow of itself is invisible ; and 
therefore, when we say we see a shadow, we 
partly hiean that we see bodies placed in 
the shadow, and illuminated by light re- 
flected from collateral bodies ; and, partly, 
that we see the confines of the light. If 
the opaque body that projects the shadow 
be perpendicular to the horizon, and the 
place it is projected on be horizontal, the 
shadow is called a right shadow; and such 
are the shadows of men, trees, buildings, 
SHA 
mountains, &c. But if the opaque body 
be placed parallel to the horizon, the sha- 
dow is called a versed shadow ; as the arms 
of a man stretched out, &c. 
“The laws of the projection of Shadows 
from opaque bodies.” l. Every opaque 
body projects a shadow in the same direc- 
tion with its rays ; that is, towards the part 
opposite to the light. Hence, as either the 
luminary or the body changes place, the 
shadow likewise changes. 2. Every opaque 
body projects as many shadows as tliere 
are luminaries to enlighten it. 3. As the 
light of the luminary is ipore intense, the 
shadow is the deeper : hence the intensity 
of the shadow is measured by the degrees 
of light that space is deprived of. 4, If a 
luminous sphere be equal to an opaque one 
it illuminates, the shadow, which this latter 
projects, will be a cylinder, and conse- 
quently will be propagated still equal to 
itself, to whatever distance the luminary is 
capable of acting; so that if it be cut in 
any place, the plane of the section will be 
a circle, equal to a great circle of the opaque 
sphere. 5. If the luminous sphere be greater 
than the opaque one, the shadow will be 
conical. If, therefore, the shadow be cut 
by a plane, parallel to the base, the plane of 
section will be a circle ; and that so much the 
less as it is a greater distance from the base. 
6. If the luminous sphere be less than an 
opaque one, the shadow will be a truncated 
cone ; and, consequently, pows still wider 
and wider ; and therefore, if cut by a plane 
parallel to the section, that plane will be a 
circle, so much the greater as it is further 
from the base. 
The sun being vastly larger than the 
Whole globe of the earth .must give all its 
shadows pointed, by reason that it illumines 
more than half ot them. In consequence 
of this demonstration we might conclude 
tliat all the sun’s shadows must be less than 
the bodies that project them, and dimi- 
nished more and more as they recede 
further and further. Now this would be 
true were there any relation between 
the body illuminated and the body illu- 
mining ; but as all objects on the earth are 
so small in comparison of that star, tlie di- 
ininution of their shadows is imperceptible 
to the eye, which sees them always equal - 
i. e. either broader or narrower than the 
body that forms them : on this account all 
the shadows caused by the sun are made in 
parallels. From the whole it appears, that 
to find the shadow of any body whatevej 
