the Sea of Serenity, to which B. and M. assigned the lumínosity 6.

If, then, we compare, the actual appearance of Linné with the text of Lohrmann and his successors, it is possible, à la rigeur, to believe that it has undergone a certain change. Linné has always been a deep crater, with elevated margins; its lustre has not changed-its total diameter has remained about the same. A comparison of maps, on the contrary, indicates a real alteration, for these figure a large crater occupying all the space now filled by the white spot. Schmidt thinks that we cannot refuse to attribute great weight to the identity of the indications of these two maps. The authors of the second, having the first at their disposal, it is probable that if they had not found the great crater drawn by Lohrmann, they would have noticed so extraordinary a fact. It is not, however, without interest to compare their indications with that of earlier maps. The picture drawn and presented by Lahire, which is in the library of St. Geneviève, represents Bessel, Sulpicius Gallus, and other little craters, equal to Linné in the map of Mädler; but he does not indicate Linné. He has only many white spots in this part of the sea. Cassini's map appears merely a copy of Lahire with less detail. According to Schmidt's note, Schröter seems not to have seen Linné,* at least not as one of the principal craters in the Sea of Serenity, although he noticed others that were smaller.

If we consult the photographs of the moon, we see, in the large copy of Warren De la Rue (1858), Bessel and Sulpicius Gallus exhibiting an indication of an interior shadow, while Linné figures as a white spot. The same is seen, though clearer, in the enlarged copy of the magnificent photograph obtained by Mr. Rutherford on the 4th March, 1865.

The disappearance of the great crater of Linné, then, dates as far back as 1858, if not as far back as Lahire. Apart from the indications supplied by the maps of Lohrmann and Beer and Mädler, to which we may oppose the counter indications of Lahire and Schröter, we only possess a single positive document testifying that Linné has undergone any change, and that is the affirmation of Schmidt that his crater and drawings of 1841 represent the object differently to what is

now seen.

REMARKS OF ELIE DE BEAUMONT.

M. de Beaumont observed, when the paper was read to the French Academy, that if observers placed in the moon viewed Vesuvius or Etna before and after an eruption, they * See Mr. Huggins's quotation from Schröter.

could only notice very slight changes. A great eruption even of Vesuvius would produce no other effect than to diminish slightly the depth of the semicircular trench of the Atrir del Cavallo, and to change its colour. Seen from the moon, such an alteration would appear problematical, and would give rise to discussions amongst observers. The observations made by P. Secchi on the 10th and 11th of February last ("Comptes Rendus," 25th February), tend materially to the belief that some change of this sort must have been produced in the configuration of the crater Linné, since the date of Lohrmann's and Beer and Mädler's maps. Moreover, it is to be desired that observations relating to the absolute permanence, or to very slight alterations on the moon's surface, should be multiplied, for a single change, however slight, would suffice to show that a geological life exists in the interior of the moon, as well as in the interior of the earth.

CURIOSITIES OF SOUND.*

PROFESSOR TYNDALL'S lectures on sound are, in their way, as admirable as the lectures on heat, which formed the foundation of his well-known work, "Heat as a Mode of Motion," though in dealing with the aerial vibrations which act upon our auditory nerves he has chiefly had to expound the discoveries of others, while in discussing the phenomena of heat it was his happy task to record many brilliant discoveries of his own. We are very glad that he has used plain English on his titlepage; a book on "Sound" promises to be intelligible and interesting, while a treatise on acoustics would look alarming and dry. All through the work before us we meet with indications of the learned professor's remarkable aptitude for presenting his subject in a simple and elegant form, and it is gratifying to be assured that the present book will do far more than has been accomplished by any preceding publication to popularize a branch of science that has suffered much neglect, from the erroneous impression that it was too abstruse for ordinary minds.

A world without sound would seem a dismal solitude to those who are familiar with human voices, the notes of birds, the cries of animals, the hum of insects, and the multitudinous noises of active life. What we call the silence of night and of waste places, and which, for a brief period, yields the

"Sound :" a Course of Eight Lectures, delivered at the Royal Institution of Great Britain, by John Tyndall, LL.D., F.R.S., Professor of Natural Philosophy in the Royal Institution, and in the Royal School of Mines. Longmans.

sensation of calmness and repose, is not as soundless as we imagine; but even that would be oppressive if endured for long; and could we visit a planet without an atmosphere such as our moon is supposed to be, how appalling would be the dreariness of its great mountain shadows, throwing their huge black pall over the scene, as the sun deserted vast regions of crags and plains, in which not the faintest whisper of any voice was heard.

We have in several previous papers explained the nature of waves, and their propagation. Sound is the result of vibrations, or wave-movements, transmitted by the air to the delicate apparatus of our ears, and then reaching our brains, where they become transformed into sensations, of which the mind takes note. In wave-motion the particles of matter first affected vibrate or oscillate through small spaces, but they communicate their own motion to other particles; and so the wave-form spreads and spreads, until it becomes too feeble to be discerned. A stone thrown in a pond illustrates these actions. Circle after circle of ripples are formed, wider and wider, but shallower and shallower, until they are stopped by the banks; or, if the pond be big enough, until, in acquiring great width, they have lost so much depth, that they can no longer be seen. Further illustration of wave-propagation, as a series of spherical shells, will be found in the paper referred to; and though the subject may appear a little difficult at first sight, it will prove very simple when approached step by step.*

If we have an instrument capable of communicating strong vibrations to the air, such as a bell, and place it under the receiver of an air-pump, and strike it while in that position, we shall have a full sound while the receiver contains its ordinary quantity of air; but keep the bell ringing, and at the same time pump the air out, the bell sounds will grow weaker and weaker, until at last, if we make the vacuum. sufficiently complete, they will no longer be heard at all. The intensity of a sound in a given medium depends on the force with which its particles are moved, or on the velocity of their motion. "Fix your attention," says Professor Tyndall," upon a particle of air as a sound-wave passes over it; it is urged from its position of rest towards a neighbouring particle, first with an accelerated motion, and then with a retarded one. The force which first urges it is opposed by the elastic force of the air, which finally stops the particle, and causes it to recoil. At a certain point of its excursion, the velocity of the particle is at its maximum. The intensity of the sound is proportioned to the square of this maximum velocity."

The intensity of a sound is, as we have seen from the air* See "Radiant Forces," INTELLECTUAL OBSERVER, March, 1867.

pump and bell experiment, also dependent upon the density of the air in which it is generated, growing feebler as that density is reduced. Professor Tyndall says, "Supposing the summit of Mont Blanc to be equally distant from the top of the Aiguille Verte and the Bridge of Chamouni, and supposing two observers stationed, the one upon the bridge, and the other upon the Aiguille, the sound of a cannon fired on Mont Blanc would reach both observers with the same intensity, though in the one case the sound would pursue its way through the rare air above, while in the other it would descend through the rare air below." If the cannon were fired in the dense air of the bridge, its sound would reach the top of the mountain; while if fired in the thin air of the mountain-top, it might be too weak to be heard below.

Sound grows weaker by spreading. When strong vibrations act upon a small number of air particles, they throw them into violent commotion; but when the same amount of force operates on a much larger quantity of air, the movement of each particle is less rapid, and the sound declines. By speaking through pipes we check the lateral propagation of the soundwave, and hence the voice can be heard at great distances; and it is easy, by means of apparatus now common in large places of business, to hold conversations with persons in any part of extensive premises. It is a curious instance of the force foolishly allowed to conventional habits, that this mode of communication is not applied in ordinary domestic life. It would be more rational for the parlour to adopt this quick mode of conveying its orders to the kitchen, than to ring bells to summon domestics to hear what is wanted, and then go and fetch it a proceeding involving a waste of labour and a loss of time. Barbaric ideas of grandeur always include waste; but as civilization advances, it will be seen that the most dignified thing is to act in the most rational way, and to accomplish desirable objects with the greatest economy of any sort of force.

Sound may be readily deadened by the interposition of non-elastic bodies. It would be difficult to talk through a feather bed, though easy to be heard through the wainscot panelling of a room.

Sound is capable of being reflected like light. Echoes are the result of natural or artificial arrangements, which send the waves back again, as many times as the echo repeats, and by interposing a balloon filled with carbonic acid in the way of sound-waves, they may be bent out of their diverging course, and concentrated just as a spherical lens concentrates beams of light. A parabolic reflector will send the rays of a lamp in a long narrow divergent cone for many miles across

the sea, and if a speaker or singer were placed in such an apparatus, his voice might be heard at great distances by persons situated within the boundaries of the cone.

The velocity of sound propagated through air at the freezing temperature is 1089 feet per second, and at 26.6° Cent., or 78.8° F., 1140 feet.* If the elasticity remains the same, augmentation of the density of air or gas diminishes the velocity of sounds. Hydrogen gas, which is as elastic as air, and much lighter, is consequently more favourable to the rapid transmission of sonorous effects.

At the freezing temperature, hydrogen transmits sound at the rate of 4164 feet per second, or nearly four times as fast as air, while carbonic acid does so at the much lower rate of 858 feet per second. Water conducts sound with more than four times the velocity of air; pine wood, along its fibres, ten times as fast; and iron, seventeen times as fast.

The elasticity of air is increased by raising its temperature, and as air opposes a certain resistance to the passage of a sound-wave, its temperature is actually raised a little by its stoppage of a quantity of motion, and its conversion into heat. From this cause air conducts sound a little quicker than was originally calculated by Newton, who did not take into account this curious cause of its change of temperature.

All

The ear is pleased with the regular recurrence of impulses, and with the succession of sounds, or their combination, according to certain principles of proportion. Noises as distinguished from musical sounds are wanting in regularity, and discords lack the desired proportions. sounds consist in a series of pulsations, and if they are to form musical notes, they must be quick enough to give the sense of continuity to the ear, and not too quick to be audible-a matter explained in a former paper, which we published, entitled, "Sounds we Cannot Hear."+ Sounds may be too deep for a particular human ear, or for any human ear, and they may be too shrill. It is only a small part of the entire music of nature that we can hear, but our range is fortunately considerable, being from 16 to 38,000 vibrations in a second. The lowest notes are, however, imperfect, and do not sound well alone, and the highest are above those used in orchestral compositions, the practical range being comprised between 40 and 4000 vibrations in a second.

The relation of the velocity of sound to the elasticity and density of the air, or other medium, is thus expressed :- "The velocity is directly proportional to the square root of the elasticity of the air, and inversely proportional to the square root of the density of the air."

+ Vol. viii. p. 413.