164 NATURAL THEOLOGY. 



ness of the means to the end, are not difficult to be appre- 

 hended. The rising and sinking of a fish in water, so far as 

 it is independent of the stroke of the fins and tail, can only 

 be regulated by the specific gravity of the body. When the 

 bladder contained in the body of a fish is contracted, which 

 the fish probably possesses a muscular power of doing, the 

 bulk of the fish is contracted along with it ; whereby, since 

 the absolute weight remains the same, the specific gravity, 

 which is the sulking force, is increased, and the fish de- 

 scends : on the contrarj^ when, in consequence of the relax- 

 ation of the muscles, the elasticity of the enclosed and now 

 compressed air restores the dimensions of the bladder, the 

 tendency downwards becomes proportionably less than it was 

 before, or is turned into a contrary tendency. These are 

 known properties of bodies immersed in a fluid. The enam- 

 elled figures, or little glass bubbles, in a jar of water, arn 

 made to rise and fall by the same artifice. A diving-ma- 

 chine might be made to ascend and descend upon the like 

 principle ; namely, by introducing into the inside of it an air- 

 vessel, which by its contraction would diminish, and by its 

 distention enlarge the bulk of the machine itself, and thus 

 render it specifically heavier or specifically lighter than the 

 water which surrounds it. Suppose tliis to be done, and the 

 artist to solicit a patent for his invention : the mspectors ol 

 the model, whatever they might think of the use or value of 

 the contrivance, could by no possibility entertain a question 

 m their minds, whether it were a contrivance or not. No 

 reason has ever been assigned, no reason can be assigned, 

 why the conclusion is not as certain in the fish as it is in 

 the machine — why the argument is not as firm in one case 

 as the other. 



It would be very worthy of inquiry, if it were possible to 

 discover, by what method an animal which lives constantly 

 in water is able to supply a repositor}'- of air. The expedi- 

 ent, whatever it be, forms part, and perhaps the most curi- 

 ous part of the provision. Nothing similar to the air-bladdei 



