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as much, with the same ratio of wing or propelling surface to weight; we should then require simply twenty times more power, or about fifteen horse-power.

There are, I think, few engineers who will assert such a concentration of power impossible, even when the weight of the man, and fuel for a considerable space of time, are included. Indeed, Strongfellow is said to have, years ago, built an engine and boiler complete which gave a third of a horse-power for a weight of twelve pounds, or weighing only at the rate of five hundred and forty pounds for fifteen horsepower.

Prof. Le Conte shows clearly that with birds "the ratio of weight to strength, and therefore the difficulty of rising, increases as the size or weight"; but in a machine it is well known that the above ratio decreases as the weight; or, in other words, we can get a hundred horse-power with a much smaller ratio of weight to power than is possible with a single horse-power.

Even now, using iron and steel and steam, we can concentrate power until we can match the energy of a bird; but, by the substitution of aluminum, possessing equal strength and weighing only a third as much as steel, why should we not hope to do much more than that?

Possibly as a heat-engine the animal machine is more efficient than the best steam-engine, although, nowadays, the "Cornish" can hardly be accepted as a standard of excellence; but it seems to me the problem is one of concentration of power, rather than economy. Surely Prof. Le Conte must be mistaken in thinking we can not equal the birds in this respect. He underrates the capacity of our engineers.

The size of a bird is no doubt limited by the strength of bone and muscle, but we are not confined within anything like such narrow limits. The huge dinosaur had reached Nature's limit of size for a walking-machine; but compare him with man's conception—the locomotive—weighing eighty tons and giving out the power of more than eight hundred horses! The whale appears to mark Nature's limit of size in marine animals. Compare him with our ocean-steamers, indicating fifteen thousand horse-power, and propelled by an instrument vastly more efficient than anything Nature has provided!

Why, then, should we be limited in our flying-machine to the weight or size of the largest bird? Surely, if we can produce in it an equal or greater ratio of power to weight, and can command the use of materials strong enough to stand the strain, we are not restricted as to size.

Nor are we limited in a machine to the birds' way of flying. The leverages of the muscles moving the wings are necessarily short and the strains great, but no more than in a steam-vessel are we bound down by Nature's methods. We may very likely be able, in some ways, to improve on her model.

Even were we to admit Prof. Le Conte's claim that the animal machine can "do more work, with the same weight of machinery and fuel," than anything "we may hope to devise," our engineers need not feel discouraged; for the power required in rising from the ground is manifestly much greater than that necessary for flight, and why should we not be able to take advantage of this by starting from some elevated point, and slide down and up aerial inclines after the manner of a bird? He then apparently does not need do a great deal of work, and why should a properly constructed machine?

The problem is a difficult one, but I can not help thinking Prof. Le Conte at fault in placing its solution among the "impossibles."

T. W. Mather.
New Haven, Conn., December 6, 1888.



Editor Popular Science Monthly:

Sir: Walking by a broad ditch, I saw a large, dark-colored snake gliding along the bottom under the water. The creature was coming directly toward me; but as I paused to observe its markings it stopped, raised its head to the surface, and remained motionless. Its back was marked with a pattern of criss-cross curves of pale yellow on a ground-tint precisely like that of water-soaked wood. Presently I began to throw little stones, to make it move on, but it did not stir. Meanwhile, the circles caused by my plashing missiles, the ripple-marks on the sandy bottom, the similar curves on the snake's back, and the crooked, lead-colored neck, all combined to pass off the animal for a water-logged stick. It would have been difficult at that moment to have convinced an inexperienced observer that it was a live snake.

This effect raised the question whether the creature was aware how nearly indistinguishable it had become. It is the common opinion that animals "play 'possum"—i.e., remain passive and apparently helpless under attack—for the purpose of safety, while they are entirely alert and would much prefer to run away. But is this theory credible when we take into account the immense self-control it demands? And can we credit such a seemingly stupid animal as a snake with sufficient intelligence to select deliberately a mode of defense requiring so nice a perception of its own appearance as well as its surroundings? Dr. Abbott thinks that the well-known behavior of the opossum when attacked is due pimply to paralyzing fear, and he supports his theory by many careful observations. If it is true that the opossum faints with terror in the presence of danger, it seems probable that, in the case of other animals, what looks like intelligent dissimulation is really due to helpless