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the sti-uctui-e of the wiug, I will pass on now to consider the kind of action and 

 the mechanical laws which are called into i>lay during flight. 



The first requisite for flight is weight, that is, the action of gravity, which 

 XJulls a bird to the ground. 



It may seem a little strange, at first sight, that the law of gi-avity, which 

 birds in flying are using great exertion to overcome, should be actually indis- 

 pensable to flight, but if birds had not more weight than the air they could not 

 fly, for they would simply float in the air like a feather, and be at the mercy 

 of every current. 



Thus we see that weight is necessai-y for flight in order to give the power 

 of directing the course of a bu-d, and to enable it to fly against the wind. 



It is a mistake, however, to suppose that great weight is essential to flight, 

 for so long as the weight of the bird is greater than that of the air, the lighter 

 the better for sustained and continued flight, for it requires less force of wing to 

 raise and support a light bird than a heavy one, and it is to this end, as we have 

 already seen, that the bird's body is made light by several beautiful contrivances. 

 I mention this as the Duke of Aigyll, in his very interesting book the "Reign 

 of Law," in the Chapter on "Contrivance or Necessity," seems to scout the idea 

 that the air-cells of birds have any function whatever in decreasing specifio 

 gravity. Now although this is probably not the only function of the aii--ceUs, as 

 I have already mentioned, yet it is most undoubtedly one of them, and I think 

 can hardly fail to strike everybody as a most beautiful contrivance for lessening 

 the weight of a bird, and at the same time not interfering with its muscular 

 power. 



Having now seen that some degree of weight is necessary for flight, I will 

 pass on to. consider the method by which that weight is raised, sustained, and 

 caused to progress in flight. 



The instrument by which flight is accomplished is, as is well known, the 

 vidng. I have already described the animal mechanism of the wing and will now 

 attempt to point out how it acts when employed in flight. 



The wing, when in the act of flying, may be regarded as a lever of the first 

 kind ; that is to say, when the power and the weight are on different sides of the 

 fulcrum, but act in the same direction ; for in the case of a bird flying, the air 

 below the wing is the fulcrum, the body of the bii'd is the weight, and the power 

 is applied at the end of and along the wing. 



It must be observed, however, that although in mechanics forces are 

 always regarded as being applied at definite points, yet in the case of a wing 

 the power is applied throughout the whole length of the wing decreasing from 

 the point of the wing inwards ; likewise the fulcrum acts as the resisting force 

 all along the wing and not at any one particular point ; nevertheless for the sake 

 of clearly comprehending the principle the action of flight, it is quite allowable 

 to suppose all the force applied by the wing to be concentrated at the end, and 



