444 



NA TURE 



[August 29, 1907 



we have yet seen. We recommend the book not only 

 to those directly interested in the scientific aspect of 

 brewing, but also to those chemists and biologists 

 whose work in any way trends in the direction of 

 brewing or malting problems. 



Oberharzcr Ganghilder. By Dr. Phil. B. Baumgartel. 



Pp. 23 + six plates. (Leipzig : Engelmann, 1907.) 



Price 7 marks. 

 The text of this book describes the geological features 

 of the Upper Harz, and the mineral veins that, accord- 

 ing to von Kocnen, were injected into the old rocks 

 of the region as recently as Miocene times. It serves 

 as an introduction to six very beautiful photographs of 

 large rock-surfaces in the mines. The various minerals 

 of the lodes have been coloured in effective but har- 

 monious tints, so that the relations of each can be 

 traced out precisely. This combination of photographic 

 accuracv with diagrammatic clearness may serve as a 

 model for reproductions in other branches of science. 

 The old coloured geological landscapes of the days of 

 Weaver and Delabeche occur to one's mind, and 

 might thus with advantage be revived. 



G. .A. J. C. 



LETTER TO THE EDITOR. 

 {The Editor does not hold himself responsible for opinions 

 expressed by his correspondents. Neither can Jte undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for tliis or any other part of Nature. 

 No notice is tahen of anonymous communications.] 



Atmospheric Absorption of Wireless Signals. 



In the Electrical Review. May 11 and 18, the writer 

 has given curves which show that telegraph messages 

 •exchanged between Scotland and Massachusetts are 

 received on some nights with practically no absorption, 

 while on other nights and in daytime nine hundred and 

 ninety-nine one-thousandths (0-999) of the energy is 

 absorbed. 



The fact that the daylight absorption was largely re- 

 duced between two stations 150 miles apart in Brazil by 

 the use of a longer wave-length suggested that the 

 masses of ionised air which are supposed to produce the 

 absorption are broken up somewhat as clouds are. During 

 the past six months experiments have been made between 

 Massachusetts and stations at Porto Rico, Cuba, Wash- 

 ington (D.C.), and New York which seem to point to the 

 same conclusion. 



Two types of transmitting apparatus were used. 



The first was an alternating-current dynamo giving 250 

 sparks per second and generating feebly damped vifaves. 

 The energy used was between 10 and 12 kw. , and the 

 frequencies used were 200,000 per second and 81,700 per 

 second. 



Messages sent with the higher frequency from Massa- 

 chusetts were received very strongly at night-time at Porto 

 Rico and Cuba, and were officially reported on several 

 occasions as having been received by naval vessels in the 

 neighbourhood of .Alexandria, Egypt (a distance of nearly 

 4000 miles), but no messages were received during day- 

 time. The absorption comes on very suddenly, and in the 

 West Indies increases sometimes nearly a thousand-fold 

 in fifteen minutes as the sun rises. 



With the longer frequency, however, though at night 

 signals were considerably weaker, probably on account 

 of the receiving stations not being adapted for such a 

 long wave-length, the daylight signals were many times 

 stronger, and it was found possible to work in daylight 

 between Massachusetts and Cuba (a distance of nearly 

 . 1700 miles) when using the lower frequency without any 

 increase in sending power. Tests between Boston and 

 Washington now continuing for nearly six months show 

 the same phenomena, i.e. that there is great daylight 

 absorption at a frequency of 200,000, but almost no 

 absorption at a frequency of 81,700. 



The second type of apparatus used consisted of a high- 

 frequency alternator capable of giving a frequency of 



NO. 1974, VOL. 76] 



100,000, but for the purposes of this test run at a fre- 

 quency of 81,700. The open-circuit voltage at this 

 frequency is 150 volts, and its armature resistance six 

 ohms. This apparatus is used for telephoning wirelessly 

 between Brant Rock, .Massachusetts, and the City of New 

 York. .A detailed description of a similar but less 

 powerful apparatus used for telephoning between Brant 

 Rock and Plymouth, Massachusetts, will be found in the 

 Electrical Rci-iew of February 15, 22, and March i, and 

 in the American Telephone Journal of January 26 and 

 February 2. The current used in the antenna; is from 

 four to six amperes, and the speech received by the New 

 York station is approximately five or six times louder 

 than the limit of audibility. Tests have now been made 

 with this apparatus over a period of nearly a month, 

 wireless telephonic communication having been first 

 established between these points about July 17. While 

 this apparatus has not been tested for so long a period 

 as the former type, the results obtained are in substantial 

 agreement. 



If the masses of ionised air were continuous there is no 

 apparent reason why there should be less absorption with 

 a long wave-length. The above experiments seem to point 

 to the conclusion that the masses of ionised air which 

 are supposed to produce the absorption are not continuous 

 but are broken up in somewhat the same manner thar 

 water vapour is into clouds. 



The fact that the wave-lengths must be increased as the 

 transmission distance is increased in order to overcome 

 the absorption does not necessarily indicate that the 

 masses are of larger size as the distance above sea-level 

 increases, though it is possible that this is the case. 



The writer has found that the absorption at night- 

 time varies with the direction from which the waves are 

 received, and has obtained some results which seem to 

 indicate that measurements of this phenomenon may have 

 a meteorological value, and may assist in extending the 

 range of weather forecasts. 



Reginald .\. Fessenden. 



Brant Rock, Mass., .August 9. 



PR.iCTIC.iL TELEPHOTOGR.iPHY. 



■pARLY in 1881 I described in N.uure (vol. xxiil., 

 p. 334) an experimental apparatus for the elec- 

 trical transmission of pictures to a distance, in which 

 use was made of one of the sensitive selenium cells 

 devised a few months previously {ibid., p. 58). Fig. i 

 shows the arrangement diagrammatically. The 

 transmitting cylinder T is mounted upon a screwed 

 spindle, which moves it laterally through 1/64 inch 

 at each revolution ; a selenium cell S is fi.xed behind 

 the pinhole H, 1/20 inch in diatneter, and is elec- 

 trically connected through the spindle with the line 

 wires L, E ; the picture to be transmitted — about two 

 inches square — is projected upon the front surface of 

 the cylinder by the lens /. The brass receiving 

 cylinder R is of the same dimensions as T, and is 

 similarly mounted; F is a platinum stylus, which is 

 pressed vertically against the metal by the flat spring 

 G; W is a variable resistance, and B,, B^ are bat- 

 teries at the transmitting" and receiving stations 

 respectively. .A piece of paper moistened with a solu- 

 tion of potassium iodide is wrapped round R, and, 

 the pinhole H having first been brought to the 

 brightest part of the focussed picture (thereby reduc- 

 ing the resistance of S to its minimum value), the 

 resistance W is adjusted so that no current passes along 

 the " bridge " C D, which, assuming the two batteries 

 to be equal, will be the case when the resistance of 

 W is the same as that of S. If now the Se cell is 

 darkened, its resistance will be increased and a cur- 

 rent will pass through the receiver in the direction 

 C D, liberating iodine at the point of the stylus F. 



To transiTiit a picture, the two cylinders are caused 

 to rotate synchronously, at the same time moving 

 from end to end of their traverses ; in the course of 



