AIR-BLADDER—AIR-GUN. 
  
  
the form of a mattress, divided into a number of 
compartments, each air-tight; a projection at one 
end forms a bolster. Kach compartment has a 
ralve, through which the air is blown in by a 
bellows. The advantages of such beds, in point of 
cleanness, coolness, lightness, and elasticity are 
obvious. They are specially valuable in many cases 
of sickness. The travelling-cushion is another con- 
trivance of the same kind. Recently, vulcanised 
India-rubber, instead of cloth, has been used in the 
fabrication of such articles. The chief drawback to 
these contrivances is the liability to being spoiled by 
a rent or other injury. 
ATR-BLADDER, or SWIMMING-BLADDER, 
in Fishes. An organ apparently intended to aid them 
in ascending in deep water, and for the accommoda- 
tion of their specific gravity to various depths. It is 
made to serve this purpose by the increase or dimi- 
nution of its volume, according to the degree of pres- 
sure exerted upon 1t by the ribs. Its place is in the 
abdomen, under the spine; and it is very various in 
size and form in different kinds of fishes. It gene- 
rally has an opening into the cesophagus, or into the 
stomach, but apparently only for the ejection, and 
not for the admission of air.. In some fishes, it has 
no opening. The air with which the A. is filled 
appears to be the result of secretion; and in fresh- 
water fishes, consists in general almost entirely of 
nitrogen, but contains a larger proportion of oxygen 
in sea-fishes; the oxygen in deep-sea fishes having 
been found to amount to 87 per cent. The A. is 
in some fishes very small; in others, it is entirely 
wanting, particularly in fishes that are destined to 
live chiefly at the bottom of the water, as flat fishes, 
   
  
S 
Air-bladder of Carp : 
Consisting of two parts—B and C, joined by a narrow neck; 
A D, a canal communicating with cesophagus, E. 
eels, &c. ; bub there are remarkable instances of its 
absence also in species of very different habits, such 
as the common mackerel, whilst it exists in other 
species of the same genus or family. The A. of 
fishes affords the finest kind of isinglass. 
AIR-CELLS, or ATR-SACS, in Birds, are remark- 
able cavities connected with the respiratory sys- 
tem. They are distributed 
along the inside of the 
whole cavity of the chest 
and abdomen ; and in 
birds of strong wing and 
rapid flight, often send 
prolongations into the 
bones. They are con- 
nected with the extremely 
active respiratory system, 
and communicate with 
the lungs, giving an im- 
mense extension to the 
surface with which the air 
inhaled comes in contact. 
The cells in the lungs of 
  
Lungs, &ec., of Ostrich : 
a aaaa,air-cells; b b, lungs; 
¢, heart; d, stomach; 
e, intestines. 
the mammalia, into which the air is 
conveyed by minute ramifications 
of the windpipe, in order to be 
brought into contact with the 
blood distributed on their walls, Air-tubes of Insect. 
are very small ; in man, only about 
one-hundredth part of an inch in diameter.—Air- 
  
  
cells, or air-sacs, may be said to form the whole res- 
piratory apparatus in some of the lower kinds of 
animals (see ANNELIDA), whilst in others, higher in 
the scale of organisation, particularly in insects, 
air-tubes arising from these ramify throughout the 
whole body. The air-tubes of insects are formed 
of a spiral fibre within a membranous coat, like the 
spiral vessels of plants, so that they possess great 
elagticity. 
AIR-CELLS in plants are cavities containing air 
in the stems or leaves. The orifices of the inter- 
cellular passages are closed up, so as to prevent the 
juices of the plant from entering them. They are 
very variable in size, figure, and arrangement, but 
are formed according to a uniform rule in each parti: 
cular species in which they are found. They are 
large and numerous in many aquatic plants, evi- 
dently serving the purpose of buoying them upin the 
water. Besides A. of regular form, there are irregular 
cavities, also called by the same name, which seem 
to be formed by the tearing of the cellular tissue in 
the rapid growth of the plant, as in grasses and 
umbelliferous plants. 
AIR-ENGINE. See CAroric ENGINE. 
AIR-GUN, an instrument for firing bullets or 
other projectiles, by the force of compressed air 
instead of gunpowder. Various forms of construe- 
tion have been adopted. The most usual plan is to 
insert a condensing syringe in the stock of the gun. 
The piston of this syringe is worked by an apparatus 
which passes through to the exterior of the gun; 
and this working causes a small body of air to be 
condensed into a chamber. The chamber has a 
valve opening into the barrel, just behind the place 
where the bullet is lodged. The gun is loaded from 
the muzzle, as ordinary muskets or fowling-pieces ; 
and there is at that time just behind it a small body 
of highly compressed air, ready to rush out at any 
opening. This opportunity is afforded by a move- 
ment of the trigger, which opens the valve ; the 
air rushes forth with such impetuosity as to propel 
the bullet. By a certain management of the 
trigger, two or three bullets, successively and 
separately introduced, can be fired off—if firing it 
can be called—by one mass of condensed air. 
Another form of A. containg several bullets in a 
receptacle or channel under the barrel; by the 
movement of a cock or lever, one of these bullets 
can readily be shifted into the barrel; and thus 
several successive discharges can be made after one 
loading—on a principle somewhat analogous to 
that of the revolving pistol. Some varieties of A. 
have the condensing syringe detached, by which 
means a more powerful condensation of air may be 
produced ; this done, the air-chamber is replaced in 
its proper position behind the bullet in the barrel. 
Those air-guns which present the external appear- 
ance of stout walking-sticks, and are thence called 
air-canes, have a chamber within the handle for 
containing condensed air, which can be unscrewed, 
and subjected to the action of the condensing syringe. 
One inventor has devised a form of A. with two 
barrels—one of small bore for the reception of the 
bullets, and another of larger bore for-the reservoir 
of condensed air; the condensing syringe being 
within the stock of the gun. An attempt has more 
recently been made to combine the action of elastic 
springs with that of compressed air, in an A.; 
springs of gutta-percha, or of vulcanised india- 
rubber, are employed in substitution of, or in co- 
operation with, a condensing syringe. No form 
of A. hitherto made has had power enough to 
propel a bullet to any considerable distance; and 
therefore the instrument is scarcely available in 
war ; there are, however, circumstances in which 
  
  
  
     
    
   
  
   
  
   
  
     
    
   
   
  
     
    
  
  
  
  
  
  
  
   
  
  
  
  
  
   
   
   
  
  
  
   
   
  
   
    
   
   
    
   
  
     
   
   
  
  
   
    
    
  
  
  
  
  
    
  
    
   
    
    
   
   
   
   
    
   
   
   
   
  
   
   
    
   
  
  
  
   
    
  
   
   
   
   
  
  
    
    
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