as to the then climate of Europe by thinking of that of the present habitat of the elephant and rhinoceros. Even now the Bengal tiger traverses Asia as far north as latitude 52°, and the lion and tiger are frequently met with when snow and ice are present.

The tools and weapons of the man of this age were simple indeed, but no mean skill was employed in their manufacture and use. Even with our many and marvelous inventions, one of us, cast away upon some uninhabited shore, could hardly manifest more self-helpfulness. And the manner in which the dead were buried-one of the common modes of expressing a race's faith in a future life-shows the possession of some degree of spiritual development.

Such are "the earliest traces of man in Europe," the slight, sparse indications of his existence in the Tertiary or next preceding formation. excepted. These traces of man in the Diluvium belong to the period geologically the most recent indeed, yet even it is separated from our own time by a gulf of many thousands of years. The rhinoceros, primitive ox, giant-elk (Megaceros Hibernicus), and cave-bear, are prominent among the contemporaries of this primitive man, but the characteristic animal of the time was the mammoth. Hence the name of

the first age of Man-the "Age of Mammoths."

We will vainly seek in this earliest man for evidences of that creaturely perfectness which, according to the common view, he must have inherited from his first parents in paradise-the charming paradise of Genesis, of art, and of poetry. For the geologist, the fruits of the truly paradisaic epoch grew in a far remoter past, when Europe was adorned with the palm and cinnamon-tree, and all the exuberant vegetation of the middle Tertiary period, whence our peat-beds are formed; when, instead of man, the ape, or possibly a man not much superior to the ape, stood at the head of God's earthly creatures.

Having answered the question as to man's first traces in Europe, we might now bring our treatise to a close. But, to gain an adequate notion of the antiquity of our race, and of its progress during the successive ages, we proceed to a cursory review of the succeeding eras of prehistoric human existence.

THE ATMOSPHERE IN RELATION TO FOG-SIGNALING. BY JOHN TYNDALL, LL. D., F.R.S.

IN

II.-Action of Hail and Rain.

N the first part of this article it was demonstrated that the optic transparency and acoustic transparency of our atmosphere were by no means necessarily coincident; that on days of marvelous optical clearness the atmosphere may be filled with impervious acoustic clouds,

while days optically turbid may be acoustically clear. We have now to consider, in detail, the influence of various agents which have hitherto been considered potent in reference to the transmission of sound through the atmosphere.

Derham, and after him all other writers, considered that falling rain tended powerfully to obstruct sound. An observation on June 3d has been already referred to as tending to throw doubt on this conclusion. Two other crucial instances will suffice to show its untenability. On the morning of October 8th, at 7.45 A. M., a thunder-storm, accompanied by heavy rain, broke over Dover. But the clouds subsequently cleared away, and the sun shone strongly on the sea. For a time the optical clearness of the atmosphere was extraordinary, but it was acoustically opaque. At.2.50 P. M. a densely black scowl again overspread the heavens to the west-southwest. The distance being 6 miles, and all hushed on board, the horn was heard very feebly, the siren more distinctly, while the howitzer was better than either, though not much superior to the siren.

A squall approached us from the west. In the Alps or elsewhere I have rarely seen the heavens blacker. Vast cumuli floated to the northeast and southeast; vast streamers of rain descended in the west-northwest; huge scrolls of cloud hung in the north; but spaces of blue were to be seen to the north-northeast.

At 7 miles' distance the siren and horn were both feeble, while the guns sent us a very faint report. A dense shower now enveloped the

Foreland.

The rain at length reached us; falling heavily all the way between us and the Foreland. But the sound, instead of being deadened, rose perceptibly in power. Hail was now added to, the rain, and the shower reached a tropical violence, the hailstones floating thickly on the flooded deck. In the midst of this furious squall both the horns and the siren were distinctly heard; and as the shower lightened, thus lessening the local pattering, the sounds so rose in power that we heard them at a distance of 7 miles distinctly louder than they had been heard through the rainless atmosphere of 5 miles.

At 4 P. M. the rain had ceased, and the sun shone clearly through the calm air. At 9 miles' distance the horn was heard feebly, the siren clearly, while the howitzer sent us a loud report. All the sounds were better heard at this distance than they had previously been at 5 miles; from which, by the law of inverse squares, it follows that the intensity of the sound at 54 miles' distance must have been augmented at least threefold by the descent of the rain.

On the 23d of October, our steamer had forsaken us for shelter, and I sought to turn the weather to account by making other observations on both sides of the fog-signal station. Mr. Douglas, the chief-engineer of the Trinity House, was good enough to undertake the observations northeast of the Foreland; while Mr. Ayres, the

assistant engineer, walked in the other direction. At 12.50 P. M. the wind blew a gale, and broke into a thunder-storm with violent rain. Inside and outside of the Cornhill Coast-guard Station, a mile from the instruments, in the direction of Dover, Mr. Ayres heard the sound of the siren through the storm; and, after the rain had ceased, all sounds were heard distinctly louder than before. Mr. Douglas had sent a fly before him to Kingsdown, and the driver had been waiting for fifteen minutes before he arrived. During this time no sound had been heard, though forty blasts had been blown in the interval; nor had the coast-guard man on duty, a practised observer, heard any of them throughout the day. During the thunder-storm, and while the rain was actually falling with a violence which Mr. Douglas describes as perfectly torrential, the sounds became audible, and were heard by all.

To rain, in short, I have never been able to trace the slightest deadening influence upon sound. The reputed barrier offered by "thick weather" to the passage of sound was one of the causes which tended to produce hesitation in establishing sound-signals on our coasts. It is to be hoped that the removal of this error may redound to the advantage of coming generations of seafaring men.

ACTION OF SNOw.-Falling snow, according to Derham, is the most serious obstacle of all to the transmission of sound. We did not extend our observations at the South Foreland into snowy weather; but a previous observation of my own bears directly upon this point. On Christmas-night, 1859, I arrived at Chamouni, through snow so deep as to obliterate the road-fences, and to render the labor of reaching the village arduous in the extreme. On the 26th and 27th it fell heavily. On the 27th, during a lull in the storm, I reached the Montanvert, sometimes breast-deep in snow. On the 28th, with great

difficulty, two lines of stakes were set out across the glacier, with the view of determining its winter motion. On the 29th, the entry in my journal, written in the morning, is, "Snow, heavy snow; it must have descended through the entire night, the quantity freshly fallen is so great."

Under these circumstances I planted my theodolite beside the Mer de Glace, having waded to my position through snow which, being dry, reached nearly to my breast. Assistants were sent across the glacier with instructions to measure the displacement of a transverse line of stakes planted previously in the snow. A storm drifted up the valley, darkening the air as it approached. It reached us, the snow falling more heavily than I had ever seen it elsewhere. It soon formed a heap on the theodolite, and thickly covered my own clothes. Here, then, was a combination of snow in the air, and of soft, fresh snow on the ground, such as Derham could hardly have enjoyed; still through such an atmosphere I was able to make my instructions

audible quite across the glacier, the distance being half a mile, while the experiment was rendered reciprocal by one of my assistants making his voice audible to me.

The flakes here were so thick that it was only at intervals that I was able to pick up the retreating forms of the men. Still the air through which the flakes fell was continuous. Did the flakes merely yield passively to the sonorous waves, swinging, like the particles of air themselves, to and fro as the sound-waves passed them? Or did the waves bend by diffraction round the flakes, and emerge from them without sensible loss? Experiment will aid us here by showing the astonishing facility with which sound makes its way among obstacles, and passes through tissues, so long as the continuity of the air in their interstices is preserved.

A piece of mill-board or of glass, a plank of wood, or the hand, placed across the open end t' of the tunnel a b c d (see page 689), intercepts the sound of the bell, placed in the padded box P, and stills the sensitive flame k (described in the last article).

An ordinary cambric pocket-handkerchief, on the other hand, stretched across the tunnel-end produced hardly an appreciable effect upon the sound. Through two layers of the handkerchief the flame was strongly agitated; through four layers it was still agitated; while through six layers, though nearly stilled, it was not entirely so.

Dipping the same handkerchief into water, and stretching a single wetted layer across the tunnel-end, it stilled the flame as effectually as the mill-board or the wood. Hence the conclusion that the soundwaves in the first instance passed through the interstices of the cambric.

Through a single layer of thin silk the sound passed without sensible interruption; through six layers the flame was strongly agitated; while through twelve layers the agitation was quite perceptible.

A single layer of this silk, when wetted, stilled the flame.

A layer of soft lint produced but little effect upon the sound; a layer of thick flannel was almost equally ineffectual. Through four layers of flannel the flame was perceptibly agitated. Through a single layer of green baize the sound passed almost as freely as through air; through four layers of the baize the action was still sensible. Through a layer of close hard felt, half an inch thick, the sound-waves passed with sufficient energy to sensibly agitate the flame. I did not witness these effects without astonishment.

A single layer of thin oiled-silk stopped the sound and stilled the flame. A single layer of gold-beater's-skin did the same. A leaf of common note-paper, or even of foreign post, stopped the sound.

The sensitive flame is not absolutely necessary to these experiments. Let a ticking watch be hung six inches from the ear, a cambric handkerchief dropped between it and the ear scarcely sensibly affects the ticking, a sheet of oil-skin or an intensely heated gas column cuts it almost wholly off.

of the transmitted sound may be observed simultaneously with the generation and brightening of the colors which indicate the increasing thinness of the film. A very thin collodion-film acts in the same way. Acquainted with the foregoing facts regarding the passage of

VOL. VI.-44

But, though oiled-silk, foreign post, and even gold-beater's-skin can stop the sound, a film sufficiently thin to yield freely to the aërial pulses transmits it. A thick soap-film produces an obvious effect upon the sensitive flame, a very thin one does not. The augmentation

APPARATUS FOR SHOWING THE INFLUENCE OF A NON-HOMOGENEOUS ATMOSPHERE ON THE TRANSMISSION OF SOUND.