Oct. 2 1, 1S80J 



NATURE 



587 



general result was as follows ; imagine a globe, E, f, g, h, 

 in the interior of which are the telephones a and b ; let E 

 and G be the two poles, and imagine the usual meridian 

 lines and parallels of latitude. It was found as the result 

 of the experiments that the observer at C, D could deter- 

 mine with tolerable accuracy the latitude oi a sound made 

 near a, B, but that he liad no ideauiliatezicr of its longitude. 

 In a later experiment two Blake transmitters were em- 

 ployed. They were placed back to back at about five feet 

 from the ground in the open air. The receiving telephones 

 were indoors, whence the speaker could be observed. The 

 results of observation coincided with those already de- 

 scribed. In order more closely to imitate the natural 

 arrangement of the ears the transmitters were then set so 

 that the diaphragms were at 45° to each other. A sound 

 made at h here produced a feebler effect than one made 

 at F ; and aftir a few experiinoits the ear seemed to be 

 able to distinguish whether the speaker were in front of, 

 or behind the transmitters. Unfortunately the two 

 transmitters were not equally sensitive, and the ear had 

 to get accustomed to the slight inequality in the intensity 



of the transmitted sounds. Prof. Bell suggests that the 

 sensations experienced by deaf persons might be studied 

 by persons possessed of normal hearing powers by 

 purposely using transmitters of unequal power, or by 

 introducing artificial resistances into the circuits. 



It also occurred to Prof. Bell that the telephone might 

 aftbrd a means of ascertaining to what degree the human 

 €ar normally has the power of appreciating the direction 

 of sound. For this purpose a number of telephones were 

 hung up in different parts of a summer-house, and were 

 connected with a switch-board so that an interrupted 

 current from a rheotome in a distant place could be sent 

 through any one at will. A person stationed at the 

 middle of the summer-house, with his eyes closed, and 

 holding his head perfectly still, was required to indicate 

 the point from which the sound seemed to emanate. The 

 indicated direction usually differed considerably from the 

 true direction, and it was found that the observer soon 

 came to recognise each individual telephone by its par- 

 ticular timbre. To obviate this a single telephone was 

 hung up in different parts of the summer-house during 

 the absence of the observer. This was very laborious ; 



nevertheless a long series of experiments were carried out, 

 and their results carefully set down in a series of eight 

 tables. Five young men were employed as observers, the 

 power of each of their ears being previously ascertained 

 by an independent test. The experiments thus carefully 

 made and tabulated are still too few, and in Prof Bell's 

 opinion too imperfect in several respects, to admit of 

 accurate generalisation ; but some deductions are unmis- 

 takable. The tables establish beyond dispute {a) that 

 the perception of the direction of a source of sound is less 

 perfect by a single ear than by both ears ; {b) they dis- 

 prove the idea that direction cannot be appreciated by 

 monaural observation ; {c) they show that the direction of 

 sound is more accurately defined as it approximates to 

 the axial line of the ears [this entirely negatives Stein- 

 hauser's theory of binaiu-al audition] ; {d) that the indica- 

 tions are proportionately at fault as the true source is in 

 any other direction, the angular error sometimes amount- 

 ing to 180" when the source is 90° from the axial line ! (t-) 

 the perception of direction is absolutely unreliable when 

 the source of sound is at the nadir with respect to the 

 observer. It should however be remembered that in 

 experiments thus made in an apartment reflexion of the 

 sound comes into play, and partially vitiates any general 

 deductions by introducing slight though unknown comph- 

 cations. 



The method adopted by Prof. Bell to measure the 

 relative hearing power of the separate ears was as 

 follows : —Two flat coils of wire were placed upon a long 

 wooden rod which passed through their centres. One of 

 these coils, the " primary," was a fixture, and was put in 

 circuit with a battery and a vibrating interrupter in a 

 distant room. The other coil, the " secondary," was 

 joined up to a telephone. When placed close to the 

 primary the induced current produced loud sounds ; the 

 observer, holding the telephone to his ear, was then 

 directed to slide the secondary coil away from the primary 

 until he ceased to hear anything. The distance between 

 the two coils was then measured. It will be seen that 

 this arrangement anticipated to some extent the sonometer 

 of Prof. Hughes. 



We venture to hope that Prof. Bell will continue these 

 interesting researches in this promising, and hitherto 

 almost une.xplored field. S. P. T. 



THE GEOLOGY OF THE LIBYAN DESERT^ 



IN his very interesting anniversary address before the 

 Academy of Sciences in Munich Dr. Zittel has brought 

 together all the known facts concerning the geology 

 of the northern districts of Africa, in a manner which is 

 calculated to render the greatest service to his fellow- 

 workers in science. The address, with its accompanying 

 map and numerous explanatory notes, constitutes indeed 

 by far the best monograph on North African geology 

 which has yet appeared. The author not only reviews 

 the works of the various travellers who have furnished 

 materials bearing upon the question, from Browne and 

 Hornemann to Fraas, Rohlfs, and Schweinfurth, but what 

 is of far more importance, gives the results of his own 

 accurate study of the rocks and fossils collected and 

 brought home by recent investigators. The general 

 results arrived at by Dr. Zittel are as follows. 



To the east of the Nile rises a mountain range com- 

 posed of highly crystalline rocks— granite, diorite, and 

 hornblendic gneiss. The peaks of this range rise tJ 

 heights varying from 5,000 to S,ooo feet. 



The oldest stratified rocks of the district appear to be 

 of Cretaceous age. Lying upon the axis of crystalline 

 rocks, and also covering wide tracts of country to the 

 south of the Great Desert, is found the Nubian sandstone 



' ■' Ueber den geologlschen Bau der libyschen Wuste. (Festrede gehalten 

 inder offentlichen Sitzung der k. b Akademie derWissenschaften ru Mflnchen 

 zur Fcier ihres cinhundert und einundjwanzigsten Stif .ungstages.) Von Dr. 



