I I 0 11 
of gravity of the whole shall fill near the 
point on which it turns. This is the end of 
a steel axis running through the centre of 
the speculum, above which it finishes in a 
square for the convenience of fitting a rol- 
ler on it, by which it is set in motion by 
means of a piece of tape wound round the 
roller. The cup in which it turns is made 
pf agate, flint, or other hard substance, and 
a cover to the whole may be made of glass ; 
by this means an observation may be taken 
with it as well covered as open, which will 
prevent injury from the spray of the sea. 
These specula are as useful by night as by 
day ; for as the images of the stars may be 
seen in the speculum, consequently any ob- 
ject that can be seen reflected from the 
glasses of the quadrants may be observed 
by the speculum, and these are all the stars 
of the first magnitude, the planets Venus, 
Mars, Jupiter, Saturn, and the moon; so 
that by having the declinations of these 
bodies in the Nautical Almanack, or indeed 
in any ephemeris, they may be used in ob- 
servations as well as the sun. , 
Horizontal dial, that drawn on a plane 
parallel to the horizon, having its style ele- 
vated according to the altitude of the pole, 
in the place it is designed for. See Dial. 
Horizontal line, in perspective, a right 
line drawn through the principal point pa- 
rallel to the horizon ; or it is in the intersec- 
tion of the horizontal and perspective planes. 
See Perspective. 
Horizontal parallax. See Paral- 
lax. 
Horizontal plane, that which is parallel 
to the horizon of the place, or nothing in- 
clined thereto. The business of levelling is 
to find whether two points be in the hori- 
zontal plane, or how much the deviation is. 
Horizontal plane , in perspective, a 
plane parallel to the horizon passing through 
the eye, and cutting the perspective plane 
at right angles. 
Horizontal range, of a piece of ord- 
nance, is the distance at which a ball falls 
on, or strikes a horizontal plane, whatever 
be the angle of elevation or direction of the 
piece. When the piece is pointed parallel 
to the horizon, the range is then called the 
point-blank, or point-blanc range. The 
greatest horizontal range, in the parabolic 
theory, or in a vacuum, is that made with 
the piece elevated to 45 degrees, and is 
equal to double the height from which a 
heavy body must freely fall to acquire the 
velocity with which the shot is discharged. 
Thus, a shot being discharged with the ve- 
VOL. IH. 
H O R 
locity ofr feet per second; because gravity 
generates the velocity 2 g, or 32£ feet, in 
the first second of time, by falling 1 6-jV, or g 
feet, and because the spaces descended are 
as the squares of the velocities, therefore as 
V 2 
4 g 2 : v 2 :: g : — - the space a body must de- 
scend to acquire the velocity v of the shot, 
or the space due to the velocity n ; conse- 
V 2 V 2 
quen tly the double of this, or — = — is 
the greatest horizontal range with the velo- 
city v, or at an elevation of 45 degrees, 
which is nearly half the square of a quarter 
of the velocity. In other elevations, the. 
horizontal range is as the sine of double the 
angle of elevation ; so that, any other ele- 
vation being e, it will be, 
As radius 1 : sin. 2e :: x sin. 2 e, 
the range at the elevation e, with the velo- 
city v. But in a resisting medium, like the 
atmosphere, the actual ranges fall far short 
of the above theorems, in so much that with 
the great velocities the actual or real ranges 
may be less than the tenth part of the po- 
tential ranges ; so that some balls, which 
actually range but a mile or two, would in 
vacuo range twenty or thirty miles. And 
hence also it happens, that the elevation of 
the piece, to shoot farthest in the resisting 
medium, is always below 45°, aud gradually 
the more below it as the velocity is greater, 
so that the greater velocities with which 
balls are discharged from cannon with gun- 
powder, require an elevation of the gun 
equal to but about 30°, or even less. And 
the less the size of the balls is too, the less 
must this angle of elevation be, to shoot the 
farthest with a given velocity. See Gun- 
nery and Projectiles. 
HORN, in physiology, a tough, flexible, 
semitransparent substance, intended for the 
defence or covering of animals. The hol- 
low horns of the ox, goat, &c. ; the hoof, 
the horny claw and nail, and the scale- of 
certain insects, as the shell of the tortoise, 
resemble each other in chemical characters ; 
but they differ very widely from stag’s horn, 
ivory, &c. Horn is distinguished from 
bone, in being softened very completely by 
heat, either naked, or through the medium 
of water, so as to be readily bent to any 
shape, and to adhere to other pieces of horn 
in the same state. Horn contains but a 
small portion of gelatine, and in this it dif- 
fers from bone, which contains a great deal. 
Horn consists chiefly of condensed albu- 
men, combined with a small and varying 
