410 
the whole mass will be supported, whilst in 
B it will fall over on the side of H. 
A 
Fig.210.  B 
In most animals moving on solids, the centre 
is supported by variously adapted organs; 
during the flight of birds and insects it is 
suspended; but in fishes, which move in 
a fluid whose density is nearly equal to their 
specific gravity, the centre is acted upon 
equally in all directions. 
The lever —Levers are commonly divided 
into three kinds, according to the relative po- 
sitions of the prop or fulcrum, the power, and 
the resistance, or weight. The straight lever of 
each order is equally balanced when the power 
multiplied by its distance from the fulcrum 
equals the weight, multiplied by its distance, 
or P the power, and the weight, are in 
equilibrium when they are to each other in the 
inverse ratio of the arms of the lever, to which 
they are attached: the pressure on the fulcrum 
however varies. 
In straight levers of the first kind, the ful- 
crum is between the power and the resistance, 
as in fig. 211, where F is the fulcrum of the 
Fig. 211. 
lever AB; P is the power, and W the weight 
or resistance. We have P:W::BF:AF, 
hence P. AF =W. BF, and the pressure on the 
fulcrum is both the power and resistance, or 
P+W. 
In the second order of levers (fig. 212) the 
resistance is between the fulcrum and the 
power ; and, as before, P: W:: BF: AF, but 
the pressure of the fulcrum is equal to W—P, 
or the weight less the power. 
Fig. 212. 
MOTION. 
In the third order of lever the power | ts 
between the prop and the resistance (fig. 213), 
Fig. 213. 
nS 
where also P: W:: BF: AF, and the pressure 
on the fulcrum is P—W, or the power less the 
weight. . 
In the preceding computations the wees of 
the lever self is Teegleched for the sake n- 
plicity, but it obviously forms a part of the 
elements under consideration, especially with 
reference to the arms and legs of animals, 
To include the weight of the lever we have 
the following equations: P. AF+AF. } AF= 
W. BF-+BF.4 BF; in the first order where 
AF and BF represent the weights of 
portions of the lever respectively. Similarly, 
in the second order P. AF=W. BF+AF-: Fr 
and in the third order P. AF=W. BF-++BF. 
* In this outline of the theory of the lever, the 
forces have been considered as acting ver- 
tically, or parallel to the direction of the f 
of gravity. iy 
The head moving backwards and forwards 
on the atlas acts on the principle of the 
first kind of lever, the fulcrum being placed 
between the power and the resistance. The” 
tibia resting on the astragalus acts on the prin- 
ciple of the second order of lever, when the 
heel is raised by the tendo Achillis, the re 
sistance being between the power and the 
fulcrum, or between the heel and the toes. ny 
In lifting a weight by the hand and bendin 
the elbow-joint, as in fig. 214, in which p tl 
power, or biceps muscle is inserted at a betwee! 
the fulerum f, and the resistance w or b, ¥ 
act on the principle of the third order of lever 
In the latter case, however, the power, in 
stead of acting vertically, is applied obliquely 
and the lever, instead of simply resting on th 
fulcrum, turns upon a point at f, 
Instead, therefore, of estimating the } 
p and w as before, according to their reciproc 
distances from the fulcrum, we resolve 
them into two other forces by pe 
drawn from the fulcrum f to the directions 
the forces p and w. Thus, in fig. 214, | 
not be tow as b f to af, but as the pe 
diculars from fon the vertical line throt 
to that on ac the direction of the inserti 
the 1 ppt a é 
he pulley.—The principles of the si 
pulley are ieactbanedl into th mecha 
animals for the production of motion, 
change the directions of the motions of 
organs in reference to that of the + 
> a 
hMendic! 
~s 
