September 1, 1896.] 



KNOWLEDGE. 



195 



length of a musical sound or note, but there is no 

 such physical connection between the waves as there is 

 in a group of ship -waves or of wind-raised waves ; they are ■ 

 independent of one another, like the successive ridges of 

 water which roll in upon a flat shore after the bursting of 

 the breakers. Each vibration of a solid consists of a 

 forward and a backward swing. During the former the 

 air is compressed ; during the latter it is rarefied ; each 

 pulse of compressed air being followed by one of rarefied 

 air. The whole air-space round the sounding body thus 

 becomes a series of concentric shells of compressed and 

 rarefied air. The compressed parts may be called crests, 

 the rarefied parts troughs. The middle C string of a piano 

 sends out waves of which the crests are separated by 

 distances of about four and a-half feet. 



The quality or timbre of a note depends upon the 

 physical character of the individual air pulses, which in its 

 turn depends upon the manner in which the vibrating 

 body executes its swing-swang motion. Each vibration 

 of a tuning-fork compresses the air gently at first, 

 the pressure rising gradually to a maximum, and then 

 decreasing as gradually. This gives a smooth soft sound. 

 The vibration of a violin string, on the other hand, 

 after compressing the air to a maximum, releases the 

 pressure very suddenly ; hence its sharper sound. The 

 curves often used to represent sound waves are no more 

 like them, in the ordinary sense of likeness, than an 

 equation is like the curve it expresses ; and the similarity 

 of these curves to the forms of water waves sometimes 

 misleads people into a mistaken notion of the kind of 

 similarity between sound waves and water waves. A 

 shaded band is the most natural mode of representation. 



Although different sound waves travel at the same speed, 

 yet, owing to the difference of interval between the pulses 

 sent out by bodies not accurately tuned to the same note, 

 reinforcement and enfeeblement can take place between 

 sounds, much as in the case of water waves. " Beats" are 

 thus simOar to the occasional arrival of unusually large 

 breakers. 



We have considered the forms of wave front given by a 

 projectile and by a vibrating body, each immersed in a 

 deep ocean of air. A disturbance on a suflicient scale to 

 affect the atmosphere through its whole height sends out 

 a wave which has a very different form of wave front. 

 Such were the explosions which accompanied the final 

 paroxysms of the Krakatoa eruption. These sent out elastic 

 waves which must have moved the air to the highest 

 limits. Viewed as a whole, the atmosphere is a thin 

 spherical shell or envelope surrounding the solid earth. 

 The front of the great Krakatoa air wave was, therefore, 

 a ring or annulus with a nearly vertical face equal 

 in height to the height of the atmosphere. From 

 Krakatoa the pulse radiated out to all points of the 

 compass, the ring-shaped wave front attaining its greatest 

 dimensions when the wave had gone halfway round the 

 earth from its starting point. Then of necessity a con- J 

 traction of the ring began, and this continued until, at the ! 

 antipodes of Krakatoa, the disturbance was approximately 

 focussed round a vertical line. From this focus of concen- 

 trated energy the wave front again expanded to the full 

 circuit of the earth, and then, contracting, focussed once 

 more around Krakatoa. Thence once more the wave spread 

 out, and thus continued, with diminishing intensity at each 

 journey, for several complete circuits oif the globe. The 

 record of its marvellous journey was registered by baro- 

 meters in all parts of the world, the mercury column 

 responding to the variation of pressure as the air pulse 

 passed each recording station. This was a sound w-ave, 

 as its rate of travel showed, though it affected the baro- 



meter far beyond the range at which the noise of the 

 eruption was recognized. 



The audibility of a sound depends upon the amount of 

 compression of the air at the point where the wave en- 

 counters the auditor. In the above case the effect upon 

 the barometer was greater than that of sounds more locally 

 intense, owing presumably to a nearly simultaneous 

 increase in density in the whole atmospheric column above 

 the barometric station. 



The effect of wind upon the velocity of sound is easily 

 understood ; the disturbance advances more slowly, rela- 

 tively, to an auditor on the earth's surface, when the air 

 is bodUy moving in the opposite direction to the wave of 

 sound. This does not explain why sound becomes so much 

 feebler when travelling against wind, for the existence of 

 a current ought not to affect the amount of condensation 

 and rarefaction. Experiments made by Prof. Osborne 

 Reynolds show that sound going against wind is thrown 

 upwards, and passes over the head of the listener. By 

 ascending to a height from the ground, the sound can be 

 caught again. This is probably due to the circumstance 

 that the wind is stronger above than below, where it is 

 retarded by the ground ; the front of a sound wave 

 travelling against the wind being consequently tilted 

 upwards. Down wind the tilt would be downwards, and 

 the sound should therefore be kept close to the ground. 



The sound wave travels quicker in hot air than in cold, 

 the density being less for the same pressure ; consequently 

 on passing obliquely from a stratum of hot air into a 

 stratum of cold air, the front of the sound wave swings 

 round, as the roUers from the sea wheel round upon the 

 end which first reaches shallow water. The continual 

 refractions and reflections which sound undergoes when 

 the air is irregularly heated interfere greatly with the 

 carrying power of sound. Thus a hot bright day, even 

 when there is no wind, is often bad for sound ; whilst a fog, 

 which screens off the rays of the sun, often makes the air 

 more transparent to sound, for the little fog particles are too 

 small to interfere much with sound waves. Only obstacles 

 of large size cast a sound shadow, the wave-length of ordinary 

 sounds being considerable ; a man's head, for instance, 

 scarcely screens off sound at all. Were it otherwise, the 

 ears would have to work independently. Similarly, to 

 concentrate sound to a focus would need a very large lens, 

 so that the ear has no focussing arrangement corresponding 

 to the crystalline lens of the eye, and our perception of 

 the direction of sound waves remains less perfect than of 

 light waves. 



M 



LINOLEUM. 



By Dr. Geori.e McGowan. 



ANY a one has doubtless put to him- or herself the 

 question — What is linoleum, and how is it made ? 

 As this material is now so widely and universally 

 used for household purposes, a few notes from 

 an exhaustive article upon its history and manu- 

 facture, contributed by Mr. Walter F. Reid to the February 

 Number of the lournal of the Suciity of Chemical Jiuiustn/, 

 may not be without interest to many readers of Knowleix-.e. 

 Although linoleum itself is quite a modern product (the 

 first factory for it was started only thirty years ago), it has 

 had numerous precursors, the earliest of which was waxed 

 cloth or canvas. A varnish called " linoleon," containing lin- 

 seed oil, was used in the eighth century; and in 1230 — (..'., 

 during the reign of Henry 111. — we find oil being employed 

 for painting in this country. In 1(!36 a patent was granted 

 for " painting with oyle cullors upon wollen cloath, kerseys, 

 and stnffes, being pper' (?) "for hanging, and alsoe with 



