72 



NATURE 



\_Nov, 2S, 1878 



The Microphone for Military and Tonometric Purposes 



My attention has just been called to a paper on the micro- 

 phone, by Prof. W. F. Barrett, in Nature (vol. xix. p. 12), 

 in which it is asked whether the latter " has ever been tried by 

 military men to detect the mining operations of an enemy ? " Will 

 you allow me to state that this application of the microphone 

 suggested itself to me many months ago, and that I have begun to 

 make experiments both in this direction and also with a view to 

 ascertain to what extent sounds can be transmitted to a microphone 

 immersed in water. Unfortunately the pressure of other matters 

 has hindered me from completing the work^ which, however, I 

 hope shortly to be able to do. 



May I take this opportunity of saying that I have been endea- 

 vouring with some success to apply the principle of the micro- 

 phone to the counting of the beats of two slightly dissonant 

 tuning-forks. In one experiment the two forks were inverted 

 and screwed through a board above which the ends of their stems 

 protruded. A thin piece of carbon was laid over these ends and 

 the arrangement was placed in circuit with a Bell telephone. 

 The beats were loudly heard and continued audible long after 

 their direct sound had ceased. A reflecting galvanometer being 

 placed in the circuit the beats were shown by deflections of the 

 light spot, but irregularities in the current made it difficult to 

 count them satisfactorily. The forks were then screwed hori- 

 zontally into a vertical board and a screw was inserted about 

 l" from the stem of each and on the same horizontal level. A 

 small piece of carbon was laid over each stem and its adjacent 

 screw. This plan gave even better results, and admits of the 

 forks being any reasonable distance apart. The experiments 

 were tried with two forks whose vibration numbers were about 

 60, and also with a pair of octave forks with vibration numbers 

 of about 256 and 512. A small piece of copper wire was then 

 attached to each of the two large forks, and mercury cups were so 

 placed that the points of the wires were just not touching the 

 mercury surface when the forks were at rest. Both mercury 

 cups were connected to one pole of the battery, and the current 

 was arranged to branch through the forks uniting at the tele- 

 phone. The beats were very loud. This plan, however, involves 

 difficulties on account of the delicacy of adjustment required for 

 the mercury surfaces and also the amalgamation of the copper- 

 points. Several other experiments have been tried, but the 

 method first described seems worth a trial by those interested in 

 tonometry. The counting of beats, which is not an easy matter 

 for aged or unaccustomed ears, may thus be immensely facilitated, 

 while the period of the forks under observation is absolutely 

 unaffected. The carbon used was that employed for the electric 

 light, and it is probable that more carefully prepared and homo- 

 geneous material would have given better results in a galvano- 

 meter experiment. George S. Clarke 



Cooper's Hill, November 19 



The Microphone as a Receiver 



On the 3rd of June last, in a paper read before the Royal 

 Society of Edinburgh, I described an experiment which showed 

 that the microphone could be used, not only as a transmitter, 

 but also as a receiver of articulate sounds. An abstract of the 

 -paper appeared shortly after in Nature, and since then I have 

 had communications from several experimenters, stating that 

 they had failed to repeat the experiment, and asking for some 

 details regarding it, I trust, therefore, that you will kindly give 

 me space for a short explanation. 



In performing the experiment the transmitting and receiving 

 instruments which I used were precisely identical. Each was 

 merely an ordinary white porcelain jam-pot, 3 J inches in diameter 

 and 4 inches deep, half filled with gas-coke, broken into coarse 

 fragments and provided with electrodes whereby a current of elec- 

 tricity could be sent through the pieces of coke. Cinders from 

 ordinary coal, if well burned, would, of course, do equally well. 

 The electrodes were two strips of tin about two inclaes wide 

 slipped down, opposite to each other, between the cinders and 

 the sides of the jam-pot and fastened by being bent over the 

 edges and bound round the outside with a cord. When these 

 jam-pots were put in circuit like a pair of ordinary telephones, 

 and a battery of two strong Grove's cells, orfour ordinary Bunsen's, 

 included in the circuit the arrangement was complete. 



In this way I have had no difficulty in making myself and 

 others clearly hear the transmission both of singing and speaking, 

 although, as I stated in my paper, the articulation is not so dis- 



tinct as I have no doubt it will be when proper forms both of 

 transmitter and receiver are adopted. 



I may mention that since then I have found the result to be 

 greatly improved by including a stronger battery in the circuit. 



Edinburgh, November 18 James Blyth 



Wind-Pressure ^ 



I became acquainted, some years since, with the singu- 

 larly great wind-pressures registered at the Liverpool Obser- 

 vatory, and I should be rather disposed to attribute them to the 

 exceptional position of the wind-gauge than to think (as the 

 writer of an article in Nature, vol. xix. p. 25, appears to do) 

 that the gauge is erroneous. I do not remember to have seen it 

 noticed, in the recent discussions on the Cleopatra's Needle, 

 that there is probably a rapid increase of wind-pressure from 

 the ground-surface upwards. In any river the velocity is least 

 at the bottom, and near the bottom the change is rapid. Simi- 

 larly, in the great current formed by the wind, I imagine there 

 is a much less velocity near the ground than at some distance 

 above it, and less on a plain than above a hill standing out of 

 the plain. Now I believe the Bidstone Obser\'atory is on a hill, 

 with the great plain formed by the Atlantic in front of it. It is, 

 therefore, in a position in which it receives an exceptionally 

 heavy wind-pressure. The pressure on the wind-gauge is pro- 

 bably much greater than on the windows of the observatory, 

 and that, again, is probably greater than the pressure on build- 

 ings more inland, where the current near the ground has been 

 more interfered with by obstructions. On the other hand, 

 Cleopatra's Needle is in a very protected position, where I should 

 be much surprised to find that the wind-pressure ever reached 

 even 40 lbs. per square foot. 



As Mr. Dixon has referred to the case of a window to dis- 

 prove the possibility of a pressure of 80 lbs. per square foot, it 

 may be well to see whether it is really conclusive. I have not 

 at hand any formula for the resistance of a simply-supported 

 square plate, but it will not be very different from that of a 

 circular plate. Now, let p — pressiure per square inch on surface 

 of plate, t its thickness, r its radius. Then the greatest stress 



in the plate is by Grashof 's formula, / = 4 . -\p. Taking a 



plate of glass 2 feet diameter, \ inch thick, and loaded with 

 80 lbs. per square foot, we get f = 4,270 lbs. per square inch. 

 In some experiments which I made under Sir W. Fairbaim's 

 direction, the tenacity of glass was found to be from 4,200 to 

 6,QOO lbs. per square in. Hence, surprising as it may seem, 

 it is probable that a pane of glass 2 feet diameter would sustain 

 a load of 80 lbs. per square foot, uniformly distributed, without 

 breaking, or a load equivalent to that of a dense crowd of 

 people. I don't, of course, think that a window would be safe 

 if subjected to such a pressure, but it is always desirable to 

 subject general statements of this kind to exact calculation ; and 

 I think we may at least infer that well-constructed glass windows 

 would sustain a considerable wind-pressure without necessarily 

 giving way. W. C. UnwiN 



Cooper's Hill, November 17 



Was Homer Colour-blind ? 



I crave some little space for aTew remarks with regard to the 

 recently much^vexed question as to the traditional blindness of 

 Homer. 



It seems to have been overlooked that, apart from the state- 

 ment made by Herodotus ' (in his life of Homer), that in 



^ Homer, according to Herodotus, was^ bom about 167 years after the 

 Trojan war and, when still a child, adopted by his stepfather, to whom he 

 succeeded in the management of a school. At an early age, however, it 

 would seem, he set out for distant voyages and, at length, after having spent 

 some time on visiting Tyrrhenia and Iberia, when about 32 or 34 years of age, 

 lost his sight from what appears to have been some chronic disease of the 

 eyes. Previously, when at Ithaca, he is said already to have had a narrow 

 escape from that calamity. The text of this important narrative runs thus : — 



.... of fj.'kv ''\QaKr\aioi Xeyovcri, rdre ij.ev irap' eavTo7s Tv(p\co- 

 drjvai, US 54 €716 <^7jV* T6Te fihu vyirj yevecrdai, vffrepov Se iv 

 Ko\o<p(ovi rtxftXwdrjvai ; (TwofnoXoyoucn 8e fioi Kal KoAocpcivioi 



rovTois 'AiriKOfifvii) Se 4s Ko\o(puya crvvefir}, iroKiv 



voa"/i(ravra tovs ocfydaXfiovs fiTJ Svi'a<rdai Sta<pvye7y t^iv vScrov, dwd 

 Tv<p\cii6vi'at fVTavda. 'Ek Se t^j- KoAo(pwyos TV(p\bs iwi/ diriKueeTai 

 eU rrji/ 'Zixvpvav ... 1 



From Colophon he sailed to Smyrna, where, for his sustenance, he began 

 and, afterwards, continued, during his long wanderings, and for a good 

 many years, the recital of his verses. 



