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Now the power needed for the minimum speed increases more rapidly than the weight of the machine.
So the larger aeroplane,which weights sixty-four times as much as the smaller,needs one hundred and twenty-eight times its horsepower to keep up.
Applying the same principles to the birds, we find that the limit to their size is soon reached.
An angel whose muscles developed no more power weight for weight than those of an eagle or a pigeon would require a breast projecting for about four feet to house the muscles engaged in working its wings,while to economize in weight,its legs would have to be reduced to mere stilts.