Principle of the Dome.
We are all familiar with Domes, especially when the Dome of St. Paul’s is the most conspicuous object in our metropolis. Few persons, however, except professional architects and builders, seem to ask themselves the principle on which the Dome is constructed.
The strength of the arch is well known, and the Dome is practically a number of arches, affording material support to each other, and so enormously increasing the strength of the edifice.
A good idea of the Dome principle may be formed by taking two croquet hoops, placing them at right angles to each other, tying them together at the intersection, and pushing the ends in the ground. Even by this very simple arrangement considerable strength can be obtained; but, if the hoops be sufficiently multiplied to form a close Dome, it will be evident that the strength will be correspondingly increased.
So strong, indeed, is the Dome, that it could be made without mortar or cement, although, of course, its strength is increased by their use. A very good example of a Dome thus constructed is found in the “igloo,” or snow-hut of the Esquimaux, which has already been described.
As to the example which I have selected, it would have been easy enough to have chosen one of the great Domes of the world, such as St. Peter’s at Rome, St. Maria del Fiore at Florence, St. Paul’s of London, or St. Geneviève or the Invalides of Paris.
I have, however, selected the present example on account of the thinness of its walls, the fragility of its material, and the enormous pressure which it has to undergo. This is the “Receiver” of the Air-pump. It is made of glass not thicker than an ordinary tumbler, and yet, even when exhausted of air, it will resist the pressure of the atmosphere for days together.
When it is remembered that the Receiver is deprived of its internal air, and therefore has to resist a pressure equal to fifteen pounds on every square inch of its surface, it may be imagined how strong the Dome is. Were the top or either side to be flat, it would be crushed as soon as a vacuum was formed sufficient to deprive it of the support of the air within.
A glance at the illustration will show how the Receiver is modelled on the same plan as the Human Skull, the outlines being curiously similar. It is this formation which imparts such strength to so thin a set of bones as those which compose the human skull as enables them to protect a sensitive organ like the brain, on which both reason and life itself depend.
Eggs also form good examples of the wonderful strength obtained by this principle, their thin shells protecting the yolk and the white, as well as the chick through its progress to maturity.
The last subject in this chapter is a curious example of an evidently accidental resemblance in form.
The figure on the right of the accompanying illustration will at once be recognised as one of those Salad-dressing Bottles which try to conceal by their shape the small volume of their contents.
That on the left represents one of the many forms through which the Medusa passes before it attains its perfect form. It was long thought to be a separate creature, and was known under the scientific name of Strobila. Modern researches have, however, made the discovery that it is one of the transitional stages between the creature known as the Trumpet-hydra (Hydra tuba) and the Medusa, popularly known as Jelly-fish.
The former almost exactly resembles the Hydra of our fresh waters. It is a tiny transparent gelatinous bag—so transparent as to be scarcely perceptible, and with some thirty or forty long and delicate tentacles hanging from its open end. These tentacles are used in catching the minute creatures on which it feeds. It is fixed, and, to use Mr. Rymer Jones’s simile, looks like a beautiful silk-like pencil waving amidst the water. Its length is not quite half an inch.
That it should be identical with the remarkable form shown in the illustration seems impossible, but such is the case. Its body becomes contracted as if tied with strings, and every segment thus formed develops a set of tentacles, breaks away, and swims off in the form of a Medusa. The upper segment is exhibited as undergoing this process.
The figure is magnified so as to show the structure better, its right length being about one-third of an inch. A full and graphic history of this creature and its manifold changes may be found in Mr. Rymer Jones’s “Aquarian Naturalist.”
It is not likely that the inventor of the Salad-dressing Bottle ever saw a Hydra, but the resemblance is strangely exact.
ACOUSTICS.
CHAPTER I.
PERCUSSION.—THE STRING AND REED.—THE TRUMPET.—EAR-TRUMPET.—STETHOSCOPE.
The Science of Sound.—Rhythmical Vibrations.—The Drum.—Primitive Drums.—The Solid and Hollow Log.—The Bass Drum and Kettle-drum.—African Drums.—Gnostic Gems and the Ashanti Drum.—Tympanum, or Drum of the Human Ear, and its Mechanism.—An artificial Tympanum.—The String.—The Bow and the Harp.—The Harpsichord and the Zither.—The Bow and the Violin.—The Cricket.—The Vibrator, or Reed.—The Jew’s Harp and Harmonium.—The Cicada and its Song.—Harmonics upon Strings.—The Æolian Harp.—Harmonics upon the Trumpet.—The Trombone.—Trachea of the Swan.—The Ear-trumpet.—The Sea-shell.—The Stethoscope.—Savage Food.—The Aye-aye.—The Siren and its Uses.—Echo and Whispering Gallery.
IN a work of this nature it would be absolutely impossible, not to say out of place, to give an account of so elaborate a subject as Acoustics, i.e. the science of Sound. Suffice it to say, that all sounds are produced by the vibration of air, and that the fewer vibrations, the lower is the sound, and vice versâ.
When such vibrations are produced regularly, they form Musical sounds, but, if irregularly, the sounds can be only distinguished under the term of Noise. The earliest germ of music lies in certain savage races, who, as long as they can maintain a rhythmical beat on any resonant substance, do not particularly care what it is. A hollow tree is a splendid instrument in their opinion, but, if this cannot be had, a dry log of wood will answer the same purpose.
Some tribes, more ingenious than others, cut a deep groove upon the upper surface of a log, hollow it through this groove, and then hammer away at it to their hearts’ content. The next move was to cut off a section of the trunk of a tree, hollow it, set it on end, and then beat it on the sides.
Lastly, some one hit upon the idea that if the open upper part of the hollowed log were covered with a tightly stretched membrane, and that if the membrane, instead of the log, were beaten, the resonance would be increased. In consequence, the real Drum was invented, and seems to have existed from time immemorial in parts of the world so distant that they could not have had any communication with each other.
Take, for example, the well-known “Bass Drum” of our bands, which is shown on the right hand of the figure. We make it a very ornamental article, with frame of metal, and heraldic decorations of all kinds.
Lying against it is one of a pair of Kettle-drums, such as are always seen in mounted bands. They look very easy to play, but, if the reader will try a pair, he will soon find his mistake.
But there are savage tribes of Western Africa who make Drums of such wonderful power that their sullen roar is heard for miles around, as their slow, triple beat summons the tribe to arms like the fiery cross of the Highland clans. As to shape, lightness, and beauty, our Drums are infinitely superior to theirs, but, so far as I can gather from personal and written narratives of African travellers, none of our Drums surpass theirs in richness, depth of tone, and power of carrying sound.
Sometimes these Drums, instead of being mere cylinders, are carved into the most strange and fantastical patterns. I possess one of these curious Drums, brought from Ashanti, and carved out of a solid piece of wood.
The strange point in it is, that it represents a double head carrying, after all negro fashions, a sort of vessel upon it. One part of the head represents a human head (not that of a negro), while the other merges gradually into an eagle’s head and beak. It is, in fact, a Gnostic gem, and would pass muster as such if it had been engraved on chalcedony, cornelian, or other semi-precious stones which are employed in the seal-engraver’s art.
Upon this composite head is placed the Drum itself, which is also cut out of the solid block, and which, after the fashion of West African Drums, has a hole on one side.
This remarkable instrument was given to me by an old merchant captain, who brought it himself from West Africa, and who, when I made his acquaintance, had actually painted it all kinds of colours, planted it in his garden, and was using the Drum as a flower-pot. Of course, as soon as it came into my possession, I put it in “pickle,”—i.e. a strong solution of alkali,—brushed off the paint, and placed it in my museum, where it is now.
On the left hand of the illustration on page [514] is given a sort of map or chart of the human Ear, with its internal Drum, or Tympanum, as it is scientifically termed.
It is by the vibration of this Drum that hearing is made possible, the vibrations of the air being transmitted to the Drum by means of a beautiful bony apparatus, termed the Hammer, Anvil and Stirrup. Sometimes the action of the Drum is partially checked, and then the sufferer is said to be “hard of hearing.” Sometimes it is broken, or its action totally clogged, and then he is said to be “stone deaf.” There have been cases where an artificial tympanum has been inserted, and answered its purpose fairly well.