at another, in which it may be distinctly heard, while it is inaudible in other positions. The dome of St. Paul's Cathedral, in London, is an example. [The principle of the whispering gallery at St. Paul's may be easily understood by referring to the accompanying diagram. If a sound, such as the tick of a watch, or a low whisper, emanates from the point, A, it may proceed from A to C, and from C to B, by a number of lesser reflections from de, C, b a alternately, terminating at B. Thus a sound that could not be conveyed directly from A to B may be heard distinctly by accumulated reflection from several points in the circular surface, A C B.]

Fig. 204.

Echoes are reflected sounds. Thus, if a person stands in front of a vertical wall, and at a distance from it of about 624 feet, if he utters a syllable, he will hear a sound which is the echo of it. If there be a series of such vertical obstacles at suitable distances, the same sound may be repeated many successive times. A good ear can distinguish nine distinct sounds in a second; and, as a sound travels 1,120 feet in the same time, for the echo to be clearly distinguished from its original sound, it must travel 125 feet in passing to and from the reflecting surface, that is, the reflector must be at least 62 feet distant.

Remarkable echoes exist in several places. One near Milan repeats a sound thirty times. The ancients mention one which could repeat the first verse of the Eneid eight times. On the banks of rivers-as, for example, on the Rhine-sounds are often echoed from the rocks, rebounding from side

to side.

[Sounds are multiplied by

E

E

B

reflection, and

the know- A

B

ledge of this.

fact is made
use of in our
large mercan-
tile houses

B

A

and other es

Fig. 205.

tablishments,

Fig. 206.

where messages are conveyed from one part of a building to another part, or even from one building to another, by means of tubes. It is probable that this was the means used by Roger Bacon, and others, to make persons unacquainted with acoustics believe that the inanimate figures spoke through their means. One of the most ingenious and celebrated of these scientific deceptions was that exhibited in Paris and London many years ago. It was invented by M. Charles, and styled the "Invisible Girl." Many solutions of this scientific enigma were attempted; but the following may be relied upon, as the editor has many times acted the part of the "Invisible Girl." apparatus consisted of a wooden frame (B B B, Fig. 205), which was supported upon four pillars (A A A A, Fig. 206). Through one of these pillars

The

a tube passed underneath the floor to an adjoining room, from which the "Invisible Girl" could see all that passed, and hear all questions, because the construction of the apparatus favoured it. To each pillar, A, was attached a bent wire, E, which terminated above in a point, and to each of these bent wires was attached a narrow silk ribbon or cord, D D, which supported a hollow copper ball, C, with four trumpets, TTT T, issuing from it at right angles. The peculiarity consisted in having the copper ball, which was supposed to conceal the "Invisible Girl," suspended by ribbons which were barely sufficient to support the weight of the ball. When a question was proposed, the sound passed through the mouth of one of the trumpets to the copper ball, and thence through another of the trumpets (T'T", Fig. 205), along canals in the frame (B), to the tube passing through the pillar (A, Fig. 206), to the room in which the person was concealed. This apparatus was exhibited at the Royal Polytechnic Institution many years ago.] Speaking-trumpets depend on the reflection of sound. The divergence is

Fig. 207.

prevented by the sides of its tube; and if the instrument is of a suitable figure, the rays of sound issue from it, as seen in Fig. 207, in a parallel direction. Its efficiency depends on its length. It is stated that through such

an instrument, from eighteen to twenty-four feet long, a man's voice can be heard at a distance of three miles. Under common circumstances, the greatest distances at which sounds have been heard are usually estimated as follow:The report of a musket, 8,000 paces; the march of a company of soldiers at night, 830 paces; a squadron galloping, 1,080; the voice of a strong man in the open air, 230. But the explosions of the volcano of St. Vincent were heard at Demerara, 345 miles; and at the siege of Antwerp, the cannonading was heard in the mines of Saxony, 370 miles.

The hearing-trumpet is for the purpose of collecting rays of sound by reflection, and transmitting them to the ear.

[The art of ventriloquism appears to depend, in some degree, on the reflection of sounds within the mouth. Professor Dugald Stewart attributed the talent of exciting the perception of articulate sounds, in such a manner as to give them the effect of emission from various distances and directions, wholly to deception and the power of imitation.]

SECOND DIVISION.

SECTION VIII.-PROPERTIES OF LIGHT.--OPTICS.

CHAPTER XXXIV.

PROPERTIES OF LIGHT.

Theories of the Nature of Light-Sources of Light-Phosphorescence— Temperature of a red Heat-Effects of Bodies on Light-Passage in straight Lines-Production of Shadows-Umbra and Penumbra.

HAVING treated of the mechanical properties of gases, liquids, solids, and the laws of motion, we are now led to the consideration of certain agents or forces-light, heat, electricity. These, by many philosophers, are believed to be matter in an imponderable state; they are therefore spoken of as imponderable substances. By others their effects are regarded as arising from motions or modifications impressed on a medium everywhere present, which passes under the name of THE ETHER.

Applying these views to the case of light, two different hypotheses respecting its constitution obtain. The first, which has the designation of the theory of emission, regards light as consisting of particles of amazing minuteness, which are projected by the shining body in all directions, and in straight lines. These, impinging eventually on the organ of vision, give rise to the sensation which we speak of as brightness or light. To the other theory the title of undulatory theory is given; it supposes that there exists throughout the universe an ethereal medium, in which vibratory movements can arise somewhat analogous to the movements which give birth to sounds in the air; and these passing through the transparent parts of the eye, and falling on the retina, affect it with their pulsations, as waves in the air affect the auditory nerve, but in this case give rise to the sensation of light, as in the other to sound.

There are many different sources of light-some astronomical and some terrestrial. Among the former may be mentioned the sun and the starsamong the latter, the burning of bodies, or combustion, to which we chiefly resort for our artificial lights, as lamps, candles, gas flames. Many bodies are phosphorescent; that is to say, emit light after they have been exposed to the sun or any shining source. Thus oyster-shells, which have been calcined with sulphur, shine in a dark place after they have been exposed to the light, and certain diamonds do the same. So, too, during processes of putrefaction, or slow decay, light is very often emitted, as when wood is mouldering, or meat is becoming putrescent. The source of the luminousness, in these cases, seems to be the same as in ordinary combustions; that

is, the burning away of carbon and hydrogen under the influence of atmospheric air;—but, in certain cases, the functions of life give rise to an abundant emission of light, as in fireflies and glowworms: these continue to shine even under the surface of water, and there is reason to believe that the phenomenon, to a considerable extent, is subject to the volition of the animal. All solid substances, when they are exposed to a certain degree of heat, become incandescent, or emit light. When first visible in a dark place, this light is of a reddish colour; but as the temperature is carried higher and higher it becomes more brilliant, being next of a yellow, and lastly of a dazzling whiteness. For this reason, we sometimes indicate the temperature of such bodies, in a rough way, by reference to the colour they emit: thus we speak of a red heat, a yellow heat, a white heat. I have recently proved that all solid substances begin to emit light at the same degree of heat, and that this answers to 977° of Fahrenheit's thermometer; moreover, as the temperature rises, the brilliancy of the light rapidly increases, so that at a temperature of 2600° it is almost forty times as intense as at 1900°. these high temperatures an elevation of a few degrees makes a prodigious difference in the brilliancy. Gases require to be brought to a far higher temperature than solids before they begin to emit light.

At

Non-luminous bodies become visible by reflecting the light which falls on them. In their general relations, such bodies may be spoken of as transparent and opaque. By the former we mean those which, like glass, afford a more or less ready passage to the light through them; by the latter such as refuse it a passage. But transparency and opacity are never absolutethey are only relative. The purest glass extinguishes a certain amount of the rays which fall on it, and the metals which are commonly looked upon as being perfectly opaque allow light to pass through them, provided they are thin enough. Thus gold leaf spread upon glass transmits a greenishcoloured light.

The rays of light, from whatever source they may come, move forward in straight lines, continuing their course until they are diverted from it by the interposition of some obstacle, or the agency of some force. That this rectilinear path is. followed may be proved by a variety of facts. Thus, if we intervene an opaque body between any object and the eye, the moment the edge of that body comes to the line which connects the object and the eye, the object is cut off from our view. In a room into which a sunbeam is admitted through a crevice, the path which the light takes, as is marked out by the motes that float in the air, is a straight line.

By a ray of light we mean a straight line drawn from the luminous body, marking out the path along which the shining particles pass.

A shining body is said to radiate its light, because it projects its luminous particles in straight lines, like radii, in every direction, and these falling on opaque bodies, and being intercepted by them, give rise to the production of shadows.

If the light is emitted by a single luminous point, the boundary of the shadow can be obtained by drawing straight lines from the luminous point to every point on the edge of the body, and producing them. Thus, let a, Fig. 208, be the luminous point, be the opaque body; by drawing the lines, a b, a c, and producing them to d and e, the boundary and figure of the shadow may be exhibited.

But if the luminous body, as in most instances is the case, possesses a

sensible magnitude; if it is, for example, the sun or a flame, an opaque body will cast two shadows, which pass respectively under the names of the umbra and penumbra-the former being dark, and the latter partially

Fig: 208.

e

illuminated. This may be illustrated by Fig. 209, in which a bis the flame of a candle, or any other luminous source, having a sensible magnitude, cd the opaque body. Now the straight lines, a cf, a d'h, drawn from the top of the flame to the edges of the opaque body and produced, give the shadow for that point of the

flame; and
the lines b c e,
bd g, drawn
in like man-

ner from the bottom of the flame, give the shadow for that point. But we see that the space between g and h, which belongs to the shadow for the top of the flame, is not perfectly dark, because it is so situated as to be partially illuminated by the bottom of the flame-and a similar remark may be made as respects the space, fe, which receives light from the top of the flame. But the remaining space, fg, receives no light whatever it is totally dark-and we therefore call it the umbra, while the partially illuminated regions, fe and g h, are the penumbra.

Fig. 209.

CHAPTER XXXV.

OF THE MEASURES OF THE INTENSITY AND VELOCITY OF LIGHT.

Conditions of the Intensity of Light-Of Photometric Methods-Rumford's Method by Shadows-Ritchie's Photometer-Difficulties in Coloured Lights -Masson's Method-Velocity of Light determined by the Eclipse of Jupiter's Satellites-The same by the Aberration of the Fixed Stars.

By Photometry* we mean the measurement of the brilliancy of light-an operation which can be conducted in many different ways.

It is to be understood that the illuminating power of a shining body depends on several circumstances. First, upon its distance-for near at hand the effect is much greater than far off-the law for the intensity of light in this respect being, that the brilliancy of the light is inversely as the square of the distance. A candle two feet off gives only one-fourth of the light that it does at one foot; at three feet it gives only one-ninth, &c. Secondly, it depends on the absolute intensity of the luminous surface: thus we have seen that a solid, at different degrees of heat, emits very different amounts of light; and in the same way the flame of burning hy

* This term is derived from the two Greek words, phose (pwç), light, and metron μετρον), a measure.