412 
From this table it appears conclusive that no 
muscular force is employed or required to pro- 
the leg forwards after it has been raised and 
nt by the flexor muscles, and that the force 
of the earth’s gravity alone on the leg is suffi- 
cient to Siiceaption that purpose. The diffe- 
rence found between the oscillation of the legs 
in the living and the dead body is very small, 
and is attributed by those authors to the elas- 
ticity of the ligaments connecting the leg to the 
trunk, and some trifling differences in the 
length of the legs, but decidedly not to muscu- 
Jar action. An application of the principles 
of the pendulum to the legs of animals mov- 
ing in a vertical plane shows that the durations 
of their periodic oscillations must be respec- 
tively as the square roots of their lengths,* 
estimated by the distance of the centre of 
oscillation ; or thetime of a complete oscillation 
of any leg from behind forwards when inu- 
fluenced only by gravity is to the time in 
which a heavy body would fall through half 
the length of the leg, considered as a com- 
pound pendulum, as the circumference of a 
circle is to its diameter. 
It further results from the periodic move- 
ments of the legs being subordinate to the 
force of gravity, that the same individual 
would necessarily walk slower as he ap- 
proached the equator, and quicker as he ap- 
roached the poles, all other circumstances 
foie equal. For example, let us suppose 
any two persons to be walking in different 
latitudes, whose legs are of unequal length, and 
acted on by unequal gravitating forces, then by 
the theory of the prema the time of the 
swinging forward of their legs respectively will 
be as the square roots of their lengths directly 
and as the square roots of the gravitating forces 
in those latitudes inversely.+ 
Mechanical effects of fluids on animals im- 
mersed in them.—When a body is immersed in 
any fluid whatever, it will lose as much of its 
weight relatively as is equal to the weight of 
the fluid it displaces. In order to ascertain 
whether an animal will sink or swim, or be sus- 
tained without the aid of muscular force, or to 
estimate the amount of force required that the 
animal may either sink or float in water, or fly 
in the air, it will be necessary to have recourse 
to the specific gravities both of the animal 
and of the fluid in which it is placed. 
The specific gravities or comparative weights 
of different substances are the respective weights 
* Let? and / equal the lengths of any two 
pendulums, ¢ ¢’ the times of vibrations, g the force 
of gravity, w tos the ratio of the circumference of 
a circle to its diameter, then 
t= a/ omits rt 
& & 
*. & 
* 22 3:3: Jt LIE x ws0) 00 CBs) 
+ Forasto=-r A/a mies a/> 
hence ¢: ¢’:: a/ =: IN pee 
MOTION, 
of equal volumes of those substances.* Wher 
any solid body is immersed in a fluid and left 
to itself, it will sink if its ific gravity is 
greater than that of the fluid; but if its specific 
gravity be less than that of the fluid, it will rise 
to the surface and be sustained there; and when 
the ppenite gravity of the solid and fluid ar 
equal, the body will remain stationary wherever 
it is placed. When the weight of any 
taken in a fluid is subtracted from its ’ 
out of the fluid, the difference is the weight o! 
a volume of the fluid equal to that of the solid; 
this is to its weight in air, as the specific gravity — 
of the fluid to that of the solid; so that generally 
the specific gravities of solid bodies are as their 
weights in the air directly, and their losses ix 
water or any other fluid inversely. > i 
The specific gravity of air, water, and m 
cury, when the barometer stands at 30 in. 
the thermometer at 55°, being to each other a: 
13, 1000, 13600, it results that all those ani- 
mals whose specific gravities approximat oO 
that of water are nearly 1000 times heavier 
than air, and more than thirteen times lighter 
than mercury, and consequently animals that 
would sink and perish in water could walk on 
ge surface of mercury. ra 
he human body in a healthy state, with 
the chest filled wih, air, is ek ighter 
than water, and its sinking generally depends 
upon the air in the lungs escaping ur der 
the pressure of water upon its immersion. Dr. 
Arnott remarks that if this truth were generally 
and familiarly understood, it would lead to the 
saving of more lives than all the mechanical 
life-preservers which man’s ingenuity will « 
contrive. ° pea 
Atmospheric pressure uces a great va~ 
riety of et Bal = proeetnpe res : 
If we estimate the surface of a man to be equal — 
to 2000 square inches, the pressure of the atmo- 
sphere on his body with the barometer at_ 
30 in. will amount to 30,000Ibs., or about 15 
tons; when the barometer falls from 30 to 27 
inches, the pressure is reduced from 15 to 13 
tons ; we need not, therefore, be surprised that 
variations of atmospheric pressure should be 
attended with corresponding sensations in living 
animals. hal 
The pressure of the atmosphere enables some 
animals (as we shall subsequently prove) to fix 
themselves to rocks with great force, to walk 
up the surfaces of glass windows, to sustait 
themselves in an inverted position on th 
* If W, w are the weights of two substance 
V, v their volumes, 8, —_ specific gravities 
then Sie:i oe aa 
> } 
Vv 
t Let W = the weight of the body in air, W" 
its weight in water or any other fluid, S = thi 
specific gravity of the solid, s = the specific gr 
vity of the fluid, then we shall have the followin 
proportions ; 
W — W’:W::s8:S8>3 
is 
» 
5 
« 
h w— Ww’ 
ence 6 =. Ww s teeter eeee 
w ag 
and s= Ty? sorceress CO 
> 
= 
