562 



SCIENCE. 



Science, The angle within which we cannot judge of the di- 

 Curiosities rec tj n of sounds, depends on the state of the ear, and 



, - 1 "'- a _ J varies with different individuals, and with the condition 



of the air, as well as with the nature of the sound. 



7- On the Polarisation of Sound. 



On the po- We have already seen in our article on OPTICS, that 

 laiisation of n ,\ ien \\ght passes through crystallized bodies, or is re- 

 flected at a particular angle from transparent surfaces, 

 it receives a particular modification called polarisation, 

 in virtue of which it passes more freely through tran- 

 sparent bodies, when presented to it in one way than 

 when presented to it in another. Mr. Wheatstone of 

 London, has discovered an analogous phenomenon with 

 respect to sound, which he describes as follows : 



" I connected a tuning fork with one extremity of a 

 straight conducting rod, the other end of which com- 

 - municated with a sounding board ; on causing the tun- 

 ing fork to sound, the vibrations were powerfully trans- 

 mitted ; but on gradually bending the rod the sound 

 progressively decreased, and was scarcely perceptible 

 .-when the angle was a right one. As the angle was 

 made more acute, the phenomena were produced in an 

 inverted order; the intensity gradually increased as it 

 had before diminished, and when the two parts were 

 nearly parallel, it became as powerful as it was when 

 the rod was straight. By multiplying the right angles 

 on a rod, the transmission of the vibration may be com- 

 pletely stopped. 



In these experiments, the axes of the oscillations of 

 the tuning fork should be perpendicular to the plane 

 of the moveable angles ; for if they be parallel the vi- 

 brations will still be transmitted. In order to prove 

 this, Mr. Wheatstone placed a tuning fork perpendi- 

 cularly on the side of a rectilineal rod. The vibrations 

 were therefore communicated at right angles : When the 

 axis of the oscillations of the forks coincided with the 

 .rod, the intensity of the vibrations was a maximum. 



In proportion as the axis deviated from parallelism 

 the intensity diminished, and when it became perpen- 

 dicular the intensity was a minimum. The phenome- 

 na of polarisation may be observed in many corded in- 

 struments. The cords of a harp are attached to a con- 

 ductor, which has the same direction as the sounding 

 board. If any cord be altered from its quiescent posi- 

 tion, so that its axis of oscillation shall be parallel with 

 a bridge or conductor, its tone will be full ; but if the 

 oscillations be excited, so that their axis be at right an- 

 gles to the conductor, the tone will be feeble. See the 

 Annals of Philosophy, No. xxxii. p. 37- 



8. M. Ventau's Gigantic Meteorological Eolian Harp. 



M. Ven- Captain Haas of Basle, has designated by these names 

 tau's gigan- an apparatus which emits of itself a variety of sounds 

 tic meteor. during a change of weather. Since the year 1787, he 

 ological j^d stretched above his garden fifteen iron wires, 320 

 h'arp" f eet l n g' ar >d at the distance of about two inches from 

 one another ; the largest were two lines in diameter, 

 the smallest one line, and those of intermediate size one 

 and a half line. They were situated towards the south, 

 and inclined 20 or 30 C to the horizon, being stretch- 

 ed by means of rollers properly arranged for the pur- 

 pose. Whenever the weather changes, these wires 

 sound with such loudness, that it was impossible to go 

 on with a concert in the house. The sounds some- 

 times resembled the hissing noise of water in rapid ebul- 

 lition, sometimes that of a harmonicon, and sometimes 

 that of a distant chime or an organ. 



The inventor of this curious apparatus is M. Ventau, 



provost at Burkli, not far from Basle. He sometimes Science, 

 shot at a mark from his window ; and in order that he Curiosities 

 might not go to the mark after each shot, he attached .^"^ ^ 

 to it a long wire to draw it to him at pleasure. He re- 

 marked more than once, that the wire sounded exact- 

 ly an octave ; and he found that every iron wire, 

 stretched in a direction parallel to the south, emitted 

 this tone at every change of weather. 



A brass wire did not produce any sound, nor did an 

 iron wire, when it is stretched from east to west. 



M. Dobereiner of Jena, conceives that the phenome- 

 non now described is an effect of an electro-magnetic 

 action ; and he proposes to try if the brass wire would 

 not sound when it communicates at its extremity with 

 an energetic electrometer. See Dr. Brewster's Jour- 

 nal rf Science, vol. iii. 



p. Description of the Harmonica. 



The name harmonica has been given to a combina- Description 

 lion of musical glasses, having their sizes so adjusted to of tlie nar - 

 one another, as to give the different octaves of the sounds momca> 

 which are commonly employed in music. When the 

 glasses, however, have different thicknesses, though their 

 size and their mode of vibrating be the same, they give 

 sounds proportional to their thicknesses. It is necessary 

 therefore to choose glasses whose thickness is equal 

 throughout the whole of their surface. Glasses must also 

 be chosen which have a clear and agreeable sound, and 

 which can be easily put into vibration. Thin glasses with 

 a high stalk have been commonly employed, or cups of 

 glass or porcelain. When glasses with stalks are used, 

 they must be placed vertically beside one another, and 

 in the order of the notes of the different octaves. They 

 are then tuned by pouring more or less water into them, 

 which depresses the sound more and more. When the 

 glass vibrates, the water is also thrown into a state of 

 vibration, as may be seen by the ripples which are 

 formed on its surface. Other fluids may be employed, 

 but the quantity poured in will vary with their speci- 

 fic gravity. If mercury is put into a very large ves- 

 sel, the edge of which is rubbed by the bow of a fid- 

 dle, the ripples on the surface of the mercury are very 

 large, and are kept up for a much longer time. In place 

 of using a bar, it is customary to apply the wetted fin- 

 ger. In this way the sound produced is the same 

 as that which produces the figure shown in Plate p 

 CCCCLXXXIII. Fig. 10. ccccLxxxni. 



In the harmonica thus constructed, there is neces- Yig. 10. 

 sarily a perceptible interval between the production of Rotatory 

 the different sounds. In order to remedy this disad- harmonica, 

 vantage, the cylindrical or rotatory harmonica has 

 been constructed. It consists of seven glass cups of a 

 proper size, having a horizontal axis passing through 

 their centre, and to which is given an uniform and con- 

 tinuous rotatory motion. A belt or board of leather 

 kept constantly wet, is extended over all the glasses in 

 the direction of their length, its two extremities being 

 firmly fixed, in order that the rotatory motion of the 

 glasses may not drag the leather along with them. In 

 order to bring out a note from any particular glass, we 

 have only to press the leather upon the glass, and the 

 intensity of the sound may be made to vary with the 

 degree of pressure applied. Still, however, a percepti- 

 ble interval of time elapses before the motion of vibra- 

 tion is communicated to each glass ; so that this instru- 

 ment is only suited to perform pieces of music, the 

 movement of which is very slow. 



M. Grenie has made great improvements on the har- 

 monica. Having procured cups of glass of the same 



