302 



NATURE 



\yuly 28, 1 88 1 



would remain there and form a sheet if the surface were not too 

 much agitated, except for a current or movement in the body of 

 water sufficient to maintain it in a constant state of intermixture. 

 Even when flowing in a regular channel there is a continued 

 interchange of position of the different parls of a stream, the 

 retardation of the bed caused variations in the velocity which 

 produce whirls and eddies and a general instability in the move- 

 ment of the water in different parts of the section. The result 

 being that the water at the bottom soon finds its way to tlie 

 surface, and the reverse. I found by experiments on straight 

 canals in earth and masonry that coloured water discharged at 

 the bottom reached the surface at distances varying from ten to 

 thirty times the depth. ^ 



In' natural watercourses, in which the beds are always more 

 or less irregular, the disturbance would be much greater. The 

 result is that the water at the surface of a running stream does 

 not remain there, and when it leaves the surface it carries with 

 it the needles of ice, the specific gravity of which differs but 

 little from that of the water, which combined with their small 

 size, allows them to be carried by the currents of viater in any 

 direction. The converse effect takes place in muddy streams. 

 The mud is apparently held in suspension, but is only prevented 

 from subsiding by the constant intermixture of the different parts 

 of the stream ; when the current ceases the mud sinks to the 

 bottom ; the earthy particles composing it, being heavier than 

 water, would sink in still water in times inversely proportional 

 to their size and specific gravity. This, I think, is a satisfactory 

 explanation of the manner in which the ice formed at the surface 

 finds its way to the bottom ; its adherence to the bottom, I think, 

 is explained by the phenomenon of rcgclation first observed by 

 Faraday ; he found that when the wetted surfaces of two pieces 

 of ice were pres: ed together they froze together, and that this 

 took place under water even when above the freezing point. 

 Prof. James D. Forbes found that the same thing occurred by 

 mere contact without pressure, and that ice would become at- 

 tached to other substances in a similar manner. Regelatinn 

 was observed by these philosophers in carefully airanged experi- 

 ments with prepared surfaces fitting together accurately and kept 

 in contact sufficiently long to allow the freezing together to take 

 place. In nature these favourable conditions would seldom occur 

 in the masses of ice commonly obsei-ved ; but we must admit, 

 on the evidence of the recorded experiments, that under particu- 

 lar circumstances pieces of ice will freeze together or adhere to 

 other substances in situations where there can be no abstraction 

 of heat. 



When a piece of ice of considerable size comes in contact under 

 water with ice or other substance it would usually touch in an 

 area very small in proportion to its mass, and other forces acting 

 upon it and tending to move it would usually exceed the freezing 

 force, and regelation would not take place. In the minute 

 needles formed at the surface of the water the tendency to 

 adhere would be much the same as in larger masses touching at 

 points only, while the external forces acting upon them would be 

 extremely small in proportion, and regelation would often occur, 

 and of the immense number of the needles of ice formed at the 

 surface enough would adhere to produce the effect which we 

 observe and call anchor-ice. The adherence of the ice to the 

 bed of the stream or other objects is always down stream from 

 the place where they are formed ; in large streams it is fre- 

 quently many miles below ; a large part of them do not become 

 fixed, but as they come in contact with each other, regelate and 

 form spongy masses, often of considerable size, which drift along 

 with the current and are often troublesome impediments to the 

 use of water-power. 



Water-powers supplied directly from ponds or rivers or canals 

 frozen over for a long distance immediately above the places 

 from which the water is drawn, are not usually troubled with 

 anchor-ice, which, as I have stated, requires open water up 

 stream for its formation. 



UPON A MODIFICATION OF WHEATSTONE'S 



MICROPHONE AND ITS APPLICABILITY TO 



RADIOPHONIC RESEARCHES- 

 TN August, iSSo, I directed attention to the fact that thin 

 disks or diaphragms of various materials become sonorous 

 when exposed to the action of an intermittent beam of sunlight, 



* Paper clx. in the Transactions of the Society, 1878. Vol. vii., pages 

 109, 168. 



= A paper read before the Philosophical Society of Washington, D.C., 

 June II, 1881, by Prof. Alex. Graham Bell. 



and I stated my belief that the sounds were due to molecular 

 disturbances produced in the substance composing the diaphragm 

 (Amer. Assoc, for Advancement of Science, August 27, 1880). 

 Shortly afterwards Lord Rayleigh undertook a mathematical 

 investigation of the subject, and came to the conclusion that the 

 audible effects were caused by the bending of the plates under 

 unequal heating (Nature, vol. xxiii. p. 274). This explanation 

 has recently been called in question by Mr. Preece (Royal Society, 

 March 10, 1881), who has expressed the opinion that although 



A B, Carbon supports: c, Diaphragm. 



vibrations may be produced in the disks by the action of the 

 intermittent beam, such vibrations are not the cause of the 

 sonorous effects observed. According to him the aerial disturb- 

 ances that produce the sound arise spontaneously in the air itself 

 by sudden expansion due to heat communicated from the 

 diaphragm, every increase of heat giving rise to a fresh pulse of 

 air. Mr. Preece was led to discard the theoretical explanation 

 of Lord Rayleigh on account of the failure of experiments 

 undertaken to test the theory. 



He was thus forced — by the supposed insufficiency of the 

 explanation — to seek in some other direction the cause of the 



F^'f/ £ 



A, Stiff wire ; p, Diaphragm ; c. Hearing tube ; d. Perforated handle. 



phenomenon observed, and as a consequence he adopted the 

 ingenious hypothesis alluded to above. But the experiments 

 which had proved unsuccessful in the hands of Mr. Preece were 

 perfectly successful when repeated in America under better con- 

 ditions of experiment, and the supposed necessity for another 

 hypothesis at once vanished. I have shown in a recent paper 

 read before the National Academy of Science, April 21, 1881, 

 that audible sounds result from the expansion and contraction of 

 the material exposed to the beam, and that a real to-and-fro 



