Page images
PDF
EPUB

were classed as the Central and gneissió

Sir William Logan, I applied to them bis term, · Laurentian,' and thus clearly distinguished them from the younger gneissic and micaceous crystalline rocks of the Central and Eastern Highlands, which were classed as metamorpbosed Lower Silurian.”

Logan was not a voluminous writer, and during the later years of his life writing was a great effort to him. Occasional papers from his pen have appeared in the Transactions of the Geological Society of London, in the Canadian Naturalist and the Canadian Journal, and some of these have already been referred to; but most of what he has written is to be found in the Reports of Progress annually submitted to the Government, and in that invaluable book, the Geology of Canada, wbich is, to a large extent, a digest of what is contained in the reports published previous to 1863. He sometimes expressed himself quaintly, but everything he wrote is clear and exceedingly concise.

In addition to being a Fellow of the Royal Society and of the Geological Societies of London and Paris, be was a member of numerous other learned societies both in Europe and America. At the time of his death, and for many years previous, he was one of our Vice-Presidents; but though frequently solicited to accept the office of President, be always declined,—not on account of any lack of interest in the Society, but he felt his time was too fully occupied to permit of his successfully discharging the Presidential duties. We have already alluded to some of the medals which were awarded to him ; but it may be mentioned that altogether he was the recipient of more than twenty, including two from the Royal Society.

And now, in concluding, let me say to you, my friends, if you would do bonor to the memory of that noble old man, who fought so long, so bravely, for his country, for science, for you, then honor the cause for which he fought: strive with all your might to advance the interests of that cause, and to raise up a superstructure befitting the solid foundation which Logan has laid. He himself even hoped to build the superstructure; but his anticipations were not realized, for life was not long enough, and we must take up the mantle which he has dropped.

B. J. HARRINGTON.

escond part "The The effect of "? is in two par

e all made variations upon Sound the first of

ART. VII.- On Recent Researches in Sound; by WM. B. TAYLOR.

[Continued from page 41.]

IV. The communication of Professor Reynolds "On the Refraction of Sound by the Atmosphere,” is in two parts; the first of which considers “The effect of Wind upon Sound," and the second part “The effect of variations of Temperature.” The experiments were all made in “a flat meadow of considerable extent;" and the apparatus employed “consisted of an electrical bell mounted on a case containing a battery. The bell was placed horizontally on the top of the case, so that it could be heard equally well in all directions; and when standing on the ground, the bell was one foot above the surface.” An anemometer was also used to determine the velocity of the wind. (Proceedings of the Royal Society; republished in the L. E. D. Phil. Mag., for July, 1875, vol. 1, p. 67.)

The experiments were made on four different days, the 6th, 9th, 10th, and 11th of March, 1874; and on the last two days the ground was covered with snow, which furnished an opportunity of comparing the effect of different surfaces on the range of Sound. Additional experiments were made on the 14th of March.

[1.] "On all occasions the effect of wind seems to be rather against distance than against distinctness. Sounds heard to windward [that is against the wind) are for the most part heard with their full distinctness; and there is only a comparatively small margin between that point at which the sound is perceptibly diminished, and that at which it ceases to be audible." (Phil. Mag., p. 63.)

[2.] The sound of the alarm-bell was always heard “farther with the wind than at right-angles to its direction; (contrary to the old observation of De La Roche in 1816,-which was obviously an exceptional one;] and when the wind was at all strong, the range with the wind was more than double that at right angles..... With the wind, over the grass the sound could be heard 140 yards, and over the snow 360 yards, either with the head lifted or on the ground; whereas at right-angles to the wind, on all occasions the range was extended by raising either the observer or the bell.” (p. 68.)

[3.] When the wind was light the sound beyond the distance of 20 yards, was much less audible at the ground than a few feet above it; and when inaudible in every direction at standing height, the sound could be distinctly recovered by mounting a tree. The same result was obtained by raising the alarm-bell

upon a post 4 feet high; which while materially increasing the range of the sound-even in the direction of the slight wind, in all other directions doubled the range. This is explained by Professor Reynolds, by the continual waste and destruction of the sound waves which pass along the rough surface of the ground or grass, causing the waves immediately above to diverge continually downward, to be in like manner absorbed ; the effect of which is to gradually weaken the sound more and more, as the waves proceed; so that even “when there is no wind, the distant sounds wbich pass above us are more intense than those we hear.” (p. 68.)

[4.] Whatever therefore tends to gradually bend downward the sound rays will increase their sensible range. Professor Reynolds found by observations with the anemometer that the velocity of the wind increased from the ground upward; (pp. 63, 64) and hence it must give greater rapidity to the upper portion of the sound waves in the direction in which it is blowing and cause their impulses to continually tip downward. “This was observed to be the case on all occasions. In the direction of the wind when it was strong, the sound could be heard as well with the head on the ground as when raised, even when in a hollow with the bell bidden from view by the slope of the ground; and no advantage whatever was gained either by ascending to an elevation, or raising the bell.” (p. 68.)

[5.] “ Elevation was found to affect the range of sound against the wind in a much more marked manner than at rightangles. Over the grass no sound could be heard with the head on the ground at 20 yards from the bell, and at 30 yards it was lost with the head 3 feet from the ground, and its full intensity was lost when standing erect at 30 vards. At 70 yards when standing erect the sound was lost at long intervals, and was only fainily heard even then; but it became continuous again when the ear was raised 9 feet from the ground, and it reached its full intensity at an elevation of 12 feet.” (p. 69.) The same results were obtained with snow on the ground, excepting that the sound was heard somewhat lower, being less dissipated or absorbed by the surface contact. At 160 yards the bell was inaudible-even at an elevation of 25 feet, and the sound was supposed to be hopelessly lost; but at a further elevation of 33 feet from the ground, it was again heard ; while at 5 feet lower it was lost. At the proper elevation the sound appeared to be as well heard against the wind as with it, at the same distance. These last two observations very strikingly correspond with and confirm the observations of Henry [3], and [4].

[6.] “The least raising of the bell was followed by a considerable intensifying of the sound;" and while it could be heard only 70 yards when resting on the ground, (i. e., one foot high), when set on a post 5 feet high, it could be heard 160 yards, or more than twice the distance, — the sound-beams evidently rising faster at or near the ground, than they do higher up. (p. 69.) “The intensity of the sound invariably seemed to waver, and as one approached the bell from the windward side, the sound did not intensify uniformly or gradually, but by fits or jerks." This is supposed to be the result of the more or less curved sound rays crossing each other at a small angle and producing an interference.” (p. 70.)

A subsequent experiment was made on the 14th of March, during a strong west wind, its velocity at an elevation of 12 feet being 37 feet per second, at 8 feet, 33 per second, and at one foot from the ground (there being no snow on the grass) 17 feet per second. While the results as to varying range fully confirmed the previous experiments, the raising of the bell caused the sound to be heard even better against the wind than in the direction of the wind. (p. 71.) This curious circumstance is explained by Professor Reynolds as “due to the fact that the variation in the velocity of the air is much greater near the ground, than at a few feet above it;" and when the bell is raised the rays of sound which, proceed horizontally will be much less bent or turned up than those which go down to the ground; and consequently after proceeding some distance these rays will meet or cross, and if the head be at this point they will both fall on the ear together, causing a sound of double intensity. It is this crossing of the rays also wbich for the most part causes the interference" just mentioned. (p. 71.)

Professor Reynolds concludes that “these experiments establish three things with regard to the transmission of sound: 1. That when there is no wind, sound proceeding over a rough surface is more intense above than below. 2. That as long as the velocity of the wind is greater above than below, sound is lifted up to windward and is not destroyed. 3. That under the same circumstances it is brought down to leeward, and hence its range extended at the surface of the ground. These experiments also show that there is less variation in the velocity of the wind over a smooth surface than over a rough one. It seems to me that these facts fully confirm the hypothesis propounded by Prof. Stokes; that they place the action of wind bevond question; and that they afford explanations of many of the anomalous cases that have been observed." (p. 71.)

[7.] In regard to the second part of the communication, treating of the effect of Temperature differences in refracting sound, Professor Reynolds shows that as "every degree of temperature between 32° and 70° adds approximately one foot per second to the velocity of sound," there must necessarily be an upward flexure of the rays, whenever by reason of any consid

erable increase of temperature in the lower strata of the air, the lower portion of the sound waves is projected in advance of the upper portion. (p. 71.) Atmospheric vapor also, though exercising but little direct influence on the velocity of sound, "nevertheless plays an important part in the phenomena under consideration ; for it gives to the air a much greater power of radiating and absorbing heat, and thus renders it much more susceptible of changes in the action of the sun. . . . . It is a well-known fact that the temperature of the air diminishes as we proceed upward, and that it also contains less vapor. Hence it follows that, as a rule, the waves of sound must travel faster below than they do above, and thus be refracted or turned upward.” (p. 72)

The variation of temperature will be greatest in a quiet atmosphere when the sun is shining. The report of Mr. Glaisher "On eight Balloon Ascents in 1862" showed that “The decline of temperature [upward] near the earth with a partially clear sky is nearly double that with a cloudy sky."* " During the night the variations are less than during the day. This reasoning at once suggested an explanation of the well-known fact that sounds are less intense during the day than at night. This is a matter of common observation, and has been the subject of scientific enquiry.” (p. 73.) The opinion must here be bazarded that this familiar phenomenon has first received its true and satisfactory explanation from Professor Reynolds.

Assuming that for a few hundred feet upward, the diminution of temperature on a clear summer day is 1° for each hundred feet, a horizontal sound-ray would be bent up in an arc having a radius of about 20 miles. From a cliff 235 feet high, a sound should be audible from 1; to 2 miles on the sea, and the ray should then begin to rise above the observer's head. This is shown to accord very closely with the observation of Tyndall [6]. Professor Reynolds after quoting the observation at length, remarks: “Here we see that the very conditions wbich actually diminished the range of the sound were precisely those which would cause the greatest lifting of the waves. And it may be noticed that these facts were observed and recorded by Professor Tyndall with his mind altogether unbiased with any thought of establishing this hypothesis. He was look. ing for an explanation in quite another direction. Had it not been so he would probably have ascended the mast and thus

* Mr. Glaisher remarks : “From these results we may conclude that in a cloudy state of the sky, the decline of temperature is nearly uniform up to the clouds; that with a clear sky the greatest change is near the earth, being a decline of 1° in less than 100 feet, gradually decreasing as in the general law indicated in the preceding section, till it requires 300 feet at the height of 5,000 feet, for a change of 10 of temperature." (Rep. Brit. Assoc, 1862, p. 462.) Am. JOUR. SCI.-THIRD SERIES, VOL. XI, No. 62.-FEB., 1876.

« EelmineJätka »