decomposition of the material as now manufactured. Fortunately, also, the two or three who had battled for gun cotton during the few previous years did not lose heart in consequence of this sad accident, and were not deterred from further efforts by shakes of the head and shrugged shoulders, by the "I told you so" of wise friends, the petty malignity of others who chose to constitute themselves foes, or the worse than discouragement where encouragement should have been hoped for. And thus it has come to pass, that in spite of all casualties, all discouragement, and all forebodings to the contrary, gun cotton has now attained an unassailable position as one of the safest and most efficient of explosive agents.

I think you will agree with me that we are taught by this history of gun cotton the very wholesome lesson never to allow ourselves to be deterred by difficulties and adversity, however severe, from steadily pursuing any labour to which we have once devoted ourselves as being worthy of our energies. Even if we fail to realise our anticipations of the immediate value and importance of our work, depend upon it our labour will not have been in vain, but will in time bear lasting fruit, by having contributed to the advancement of knowledge and the development of truth.

And now let me, in conclusion, endeavour to show you the difference between the ordinary burning of gun cotton and its detonation.

Professor Abel then took a disc of gun cotton and set fire to it. The cotton burned a considerable time, with an intense light and considerable heat. This experiment illustrated that gun cotton could be made to burn steadily in open air, and that it was under perfect control. The cotton burned weighed nine ounces. lecturer remarked that had as many hundredweight been ignited in a heap its burning would most probably have raised some portion rapidly to the heat at which it would explode.

The

The last experiment consisted in using a small charge (about one-fiftieth part of the weight of the disc) and firing it with a large percussion cap exploded by electricity. The result was what the lecturer termed a "respectable explosion," the concussion being a very sharp one, and a brick, upon which the little charge rested, being shattered by the explosion.

A LECTURE, Delivered in the Hulme Town Hall, Manchester, on Wednesday, November 26th, 1873.

By S. M. BRADLEY, Esq., F.R.C.S.

HERE is nothing of which we are so justly proud as our mechanical inventions, yet every time a sparrow hops from a house-top we have an illustration of animal mechanics, which both in the construction of the machine and the mechanism of its movements transcends man's mightiest achievements.

Now I purpose this evening taking a single illustration from the rich and wide domain of animal mechanics, but this perhaps the most beautiful, as it is the most complicated, of them all-I mean the flight of a bird. If the phenomenon of flight were a novelty-if we had never seen a bird fly, and were suddenly to see, say, an eagle dart through space from the bosom of the blue empyrean to the level of the green sward, there turn with the rapidity of lightning and soar straight up to the sun again, we should somewhat wonder at the liberties taken with the laws of gravitation, and should try to puzzle out how the thing was doneto find out, in a word, what the animal mechanics were. what I hope to find out, with your help, this evening.

This is

Now the first thing we notice about a machine is its shape. Let us then first examine the shape of the bird, and then the nature of the moving power.

The general shape of a bird is something like the shape of a fish; the object being the same in both, namely, to offer as little resistance as possible to the medium-whether it be air or

water-tnrough which the animal has to force its way. But if you look at a bird with the wings spread, you will find that it is more boat-like in its build. All the best flyers are shaped something like a yacht or cutter. You are perhaps familiar, most of you, with such a boat as this, with the old-fashioned lateen sail, common enough on the Mediterranean. Now, if you happen to be on a mountain-top, and see such a boat reflected in the water; or, if—which comes to the same thing-you double it by adding another to the lower part, you will find that the boat is changed into a fair resemblance of a swallow; only the object aimed atavoiding resistance, and taking advantage of the currents of airis much more perfectly achieved in the natural than in the artificial machine.

Now, let us turn to the skeleton of the bird and see what special adaptations are met with here for its aërial existence. Let us contrast for this purpose the scaffold upon which man is built with the skeleton of a bird. You will find, if you contrast the two almost superficially, that there is a strong family likeness between them; the same idea evidently runs through the construction of them both. But while they are much alike in their general construction, there are many variations in the details; they are, so to speak, not "like to like, but like in difference." Look, for instance, at the contrasted heads and necks of these two creatures. You find the head of the bird narrow and tapering and the neck very long and flexible as compared with the same parts of man, for these parts have to perform all the functions of hands and arms. Look at the shoulder of the bird. You have here a double joint; an extra bone indeed, which is absent in the skeleton of man, or at least it has dwindled down into the little coracoid process; whilst in the bird these coracoid bones form the chief elements of the shoulder, and are much larger than the little collar bones, or merrythought, which are placed in front. Then observe how solidly composed is the entire breastwork of the bird. You find in the bird that there is a prominent and strong keel on the breastbone, while the breastbone of man is quite flat. This keel serves a double purpose it gives increased space to the great muscle which moves the wing, and it also strengthens the bone. If an engineer wants to strengthen an iron plate, he places another piece at right angles to it, forming what is called T iron. Nature has adopted this form in the breastbone of all birds which are capable of flight.

If you contrast the arm-bones of the bird and of man, you will find that bone for bone they correspond; but this upper armbone, or humerus, is much longer, and relatively stronger, than the

same bone of man, having to perform more powerful mechanical feats. The bones of the fore arm very closely correspond; but they, too, in the bird, are longer and relatively stronger. If, then, you look at the bones of the wrist and the fingers, you will find that in the bird they are stunted, insignificant affairs, merely having to support certain feathers, and strongly contrast with the wonderful and complex mechanism of the human hand. The same likeness in difference pervades the whole skeleton. If now you take a human bone and cut across it, you will find that it is hollowed out in the interior; and this same construction is carried even further in the bones of the bird; in fact the bone of a bird is a mere hollow cylinder; so that, size for size, the bone of a bird is much lighter than the bone of a man, but weight for weight it is stronger, because the structure is more tightly packed.

Now we find by experiment that the human bone is relatively to weight four times as strong as cast-iron, but the bone of a bird is, weight for weight, six times as strong. This human bone required a weight of nearly 900lbs. to break it, but the bone of a bird of equal calibre sustained a weight of 1,500lbs. before it snapped. If you look further at the cut ends of the bone of a bird you will find that this hollow shaft is crossed by a most beautiful lattice-like arrangement of the structure. Now you all know how architects strengthen roofs by tie-beams, girders, and so forth, and very often you will find that there is a right-angled girder which is connected with a cross-piece, the cross-piece being prevented from yielding by a ring of metal placed between the two. You may see such a construction at Stretford station any day in the roof supporting that notable piece of architecture! Nature has adopted this plan ever since she made the first bone. But in spite of the great strength and density of the bone, it is everywhere porous like blotting-paper, so that it freely admits moisture, by means of which the bone is nourished and lives.

The thickness of the human skull is notorious, but yet it is everywhere transparent; so that it is by no means a difficult thing as I daresay Mr. Harrison will show us to see right through a man's skull. [Illustration by means of the electric light and the oxy-hydrogen lantern.]

While Mr. Harrison is preparing for the experiment, I will take this opportunity of expressing my hearty thanks for his continuous and constant aid in this lecture; in fact, whatever element of success it possesses is due to him rather than to me. I wish too to thank Mr. Searson very sincerely for the beautiful diagrams which cover the walls.

What is true of the skull is true of every bone in the body. You have therefore in this bony support [the skeleton of a bird] a structure at once light, freely movable, and of sufficient strength to resist all the ordinary forces which are brought to bear against it. But there is something else very interesting and important in the fact that the bones of birds are hollow. You know what a great quantity of fuel a high-pressure engine burns in a day; and in like manner our bird, which is a very high-pressure engine indeed, consumes a great deal of fuel in the shape of food. The swallow, e.g., will eat its own weight of food in a day; and the cormorant more than rivals the swallow in this respect. Just as the engineer requires a constant supply of fresh air to keep his fire bright and clear, so does the bird need a quantity of air to purify the blood, which is the fuel that heats the muscles to the pitch to perform their work. Now to attain this end the air passes not only into the lungs of the bird, but through them into the general cavity of the body, and even into the interior of the hollow bones; so that the warm blood is exposed on every side to the invigorating oxygen of the air. But even this is not all; for, owing to the fact that these bony cavities are connected with the respiratory or breathing apparatus, the bird can either fill these bones with air or exhaust the air from them, turning its lungs into a condensing or exhausting syringe at pleasure. Owing to this fact a high-flying rapacious bird can empty its bones of air and swoop like a cannon ball upon its prey; but ere it touches the ground it attenuates the air by allowing the air to re-enter the hollow bones, and so averts its otherwise inevitable destruction.

necessary

If we glance for one moment at the skeleton of a bird in its entirety you will find that not only is each part admirably adapted to perform its functions, but that each part is perfectly in harmony with every other part, and in a measure necessitates every other part, just as the smallest segment of a circle gives the whole figure; or as many a man in this room could, I daresay, from the piston of an engine build up the entire machine, so to one who has sufficient knowledge the smallest fragments of an animal will enable him to build up the entire organism, and to infer its habits and nature.

Let me try to give you an illustration of what I mean. Here is the beak of an eagle. Now, suppose that this were given to a man who had never seen such a bird, but who knew the general nature and habits of the class, how would he be able to say what was the character of skeleton to which this bill was appended? Well, he would know from its hooked character, that this was