F I S 
fingle effort: fiioals of one Ipecies follow thofe of 
another through vaft tradls of ocean, from the vi- 
cinity of the pole down to the equator. The cod 
purfues the whiting from the banks of Newfound- 
land to the fouthern fliores of Spain; and in the 
fame mannner, the cachalot, or blunt-headed whale, 
purfues the fhoals of herrings, and fwailov/s thou- 
faui 's at a gulp. 
This is very probably one caufe of the annual 
migration of Fifhes from one part of the ocean to 
another : but there are other motives v>^hich unite 
in caufing this ; Fifhes may be induced to change 
the place of their refidence for one more congenial 
to their conftitutions, or better adapted to depo- 
fiting their fpawn. It is worthy of obfervation,. 
that Fifhes are averfe to cold waters, and generally 
frequent thofe places where it is warmeft. In fum- 
mer, they abound in the fliallows near the Ihore, 
where the fun has power to warm the water to the 
bottom ; but, in winter, they frequent the greatelt 
depths of the fea, where the cold of rhe atmofphere 
is not fufficicndy penetrating to reach them. Frefli- 
water Fifhes are often fecn dead after fevere frofts, 
which have been killed by the fever! ty of the cold, 
or excluded from the benefit of air by the ice. 
Though all the various fpecies of Fifh live in 
the M/ater, yet they all require the afTiftance of air. 
Thofe of the whale kind breathe the air like the 
human race, and rife to the furface every two or 
three minutes for a frefh infpiration; and thofe 
which continue entirely under water mufl be fup- 
plied with air, otherwife they will expire in a very 
fhort time. When the ice covers the whole furface 
of a pond, and thus keeps off the air from the fub- 
jacent fluid, we fometimes find all the Fifh are de- 
ftroyed. If a hole be made in the ice, the Fifh 
will be feen flocking to that part, in order to re- 
ceive the benefit of a frefh fupply of the admitted 
air. Should a carp in a large vafe of water be 
placed under an air-pump, and then deprived of 
it's air, during the operation a number of bubbles 
appear on the Fifh's body; foon after, the animal 
begins to breatlie more frequently, and with greater 
diflicuky ; it then rifes to the furface, in order to 
obtain more air; the bubbles on it begin todiiap- 
pear; and the belly, which before was inflated, be- 
comes on a fudden funk, and the animal expires 
in convuifions at the bottom. 
i. nus air feems efTentially neceffary to all ani- 
mals; yet nothing is more difUcult to be accounted 
for than the manner in which Fiflies obtain this 
requifite fupply. When a Fifli is in the water, 
it is eafy to obferve the motion of it's lips and 
gills, or at leafi of the bones on each fide which 
cover them. Tliis motion in the animal is un- 
queftionably analagous to our breathing; but it is 
v/atcr and not air which the Fifh actually fucks in, 
and fpouts out through the gills at every motion. 
The manner in which it performs this operation is 
thus: the Fifh firfl admits a quantity of water by 
the mouth, which is driven to the gills; thefe clof^ 
ing, keep the water fo fwallowed from returning by 
the mouth; while the bony covering of the gills 
prevents it from pafTing through them till the ani- 
mal has extrafted the proper quantity of air from 
the body of water thus imprifoned : then the bony 
covers opening, give it a free pafTage; by which 
means alfo the gills are again opened to admit a 
frefh quantity of water. Should the Fifh be pre- 
vented from the free play of it's gills, or the bony 
covers be kept from aftion by any artificial con- 
finement, the animal would foon fall into convui- 
fions, and expire in a few minutes. 
F I S 
But though the general refpiratlon of FiHies 
may be thus explained, the difliculry remains, to 
know how this air is m.anaged which the Fifh fepa- 
rates from the water. There feems no receptacle 
for containing it; the flomach, which is the prin- 
cipal cavity within the body, is too much filled with 
aliment for that purpofe. There is, indeed, a pretty 
large cavity, called the air-bladder or fwim; but 
philofophers have long deftined this to a very dif- 
ferent purpofe. The ufe alm.ofl unlverfaily af- 
figned to the air-bladder is to enable the Fifh to 
rife or fmk in the water at pleafure, according as 
that is dilated or comprefTed. The ancients, how- 
ever, confidered the air-bladder as a fubflitute for 
the lungs, and as a ftore-houfe of air from which 
the animal might fupply it's necefTitles ; and to this 
latter opinion feveral of the moderns feem to in- 
cline. We fliall therefore exhibit both op'nions, 
with their proper fhare of evidence, and leave the 
reader to follow the theory which feems mofl ra- 
tional and fatisfactoryv The air-bladder is de- 
fer! bed as a bag filled with air, fometimes com- 
pofed of one, fometimes of two, and fometimes of 
three divifions, fituated towards the back of the 
Fifh, and opening Into the maw or gullet. Thofe 
who contend, that this bag is defigned for raifino- 
or deprefTing the Fifh in the water, build on the 
following experlm.ent: a carp being put into the 
air-pump, and the air exhaufled, the bladder ex- 
pands itfelf to fuch a degree, that at laft it burfts, 
and then the Fifli finks, and is ever after incapable 
of rifing to the furface. The air-bladder likewife 
has been pricked and wounded to let out the air; 
and on this occafion likewife the Fifh funk, and 
rofe no miore. Hence it is inferred, that the ufe 
of the air-bladder mufl be by the voluntary infla- 
tion of the animal, to increafe the furface of it's 
body ; and thus diminifhing it's fpecific gravity, to 
enable it to rife to the furface of the water, and 
keep there at pleafijre. On the contrary, when 
the Fifh is defirous to defcend, it only exhaufls this 
bladder of it's air; and the fuperficies of the Fifh 
being thus diminifhed, it confequently finks to the 
bottom. 
Such is the account given of the ufe of the air- 
bladder; but it does not appear to reft on irrefra- 
gable evidence and accurate enquiry. In the firft 
place, though nothing is more certain than that a 
carp will fwell on being put into an air-pump, yec 
a moufe or a frog will do the fame, and in thefe no 
air-bladder has ever been difcovered. A carp will 
rife to the furface, but this only evinces that the 
animal wants air ; and when it finks, it is not from 
the laceration of the air-bladder, but from it's be- 
ing totally exhaufted of air. The Fifh, indeed, 
after this experiment, will moft probably continue 
to creep at tlie bottom ; and fo will all Fifh which 
are fick and wounded, which muft be the cafe with 
this after fuch an operation. Thus the previous 
fafts prove nothing, but that when the Fifli is killed 
in an air-pump, the air-bladder is found exhaufted; 
and that muft naturally and neceffarily happen, 
for the drain of air by which the Fifh is fupplied 
in the natural way will infallibly oblige it to have 
recourfe to all it's fecret ftores; and as there is an 
evident communication between the gullet and the 
air-bladder, the air which the latter contains will 
obvioufiy be drawn away. Indeed, it is pretty 
clear, that the Fifh can neither increafe nor dimi- 
nlfh the quantity of air in this bladder, according 
to it's own pleafure, any more than mankind can 
that which is contained in their ftomachs. The 
animal has not one mufcle for contradting or di- 
4 F lating 
