Sept. 4, 1879J 



NATURE 



445 



Report of the Committee on an Instrument for Detecting Fire- 

 damp in Mines, by Prof. G. Forbes. — From the rough model 

 shown last year the Committee had constructed two new instru- 

 ments, which appeared to them to answer the purpose of measur- 

 ing the quantity of fire-damp in a coal mine. The one was of a 

 large size, and was worked by an electric battery. The other 

 was small, portable, easily worked, and it answered all the pur- 

 poses for which it was required. Both instruments were founded 

 upon the facts that sound travels quicker in light gases than in 

 dense ones, and that air which is contaminated with fire-damp is 

 lighter than pure air. The velocity of sound in different quali- 

 ties of air was compared by noting the lengths which must be 

 given to a brass tube to cause it to resound to a tuning-fork. 

 The accuracy of the instrument was such that the percentage of 

 fire-damp could be determined with an error of considerably less 

 than I per cent. On Monday the Committee were enabled to 

 descend the Wharncliffe Silkstone Colliery, in the neighbourhood 

 of Sheffield, by the kindness of the manager, Mr. George 

 Walker, who accompanied them with a number of gentlemen 

 interested in the experiments. This pit was at a depth of 200 

 yards. Mr. Walker had kindly arranged to stop the ventilation 

 and the pit at the end of the workings. After proceeding for a 

 mile through the galleries they reached this spot, where they 

 hoped to find a large amount of fire-damp. But only a slight 

 quantity was to be found, the Davy lamp generally showing but 

 a feeble blue cap, and the Forbes indicator registering only small 

 percentages. Disappointed liere, they were taken by Mr. Walker 

 to another working, where it was thought possible there might 

 be some gas. In a crevice in the roof a flow of gas was found, 

 forming a stratum of light gas. The instrument indicated quan- 

 tities which gi-adually increased, as the tube got filled with the 

 air in the crevice, from 14 to 28 per cent. But the small quan- 

 tity of gas rendered the experiment unsatisfactory, and the 

 Committee were then taken to a disused part of the mine, where 

 it was known there was a blower. Gas in sufficient quantities 

 was found, and the instrument registered gas with more readi- 

 ness than the Davy lamp. But the greatest quantity registered 

 was 6 per cent., or twelve times the smallest quantity which the 

 indicator detects. There was in the present form of the instru- 

 ment a difficulty in filling the tube with the air of the place under 

 examination, and the Committee considered that it would be 

 well to alter the instrument so as to obviate the difficulty. From 

 the experiments they could assert that this instrument was 

 capable of detecting and measuring fire-damp even in small 

 quantities. 



SECTION A— Mathematical and Physical 



On Lightning Protectors for Telegraphic Apparatus, by W. II. 

 Treece.— For many years it was not the practice in England to 

 protect telegraphic apparatus from the injurious effects of atmo- 

 spheric electricity because the damage done was so insignificant, 

 and because the remedy was found to be worse than the disease. 

 But as telegraph systems increased, as the country became enve- 

 loped in (jne vast network of wires, it was found that the damage 

 done becam-. considerable, until, in fact, about 10 per cent, of 

 the appamtus in use were in one year damaged. 



Lightning protectors then became essential. Many forms were 

 tried based on the fact that when a discharge takes place through 

 ■a non-conductor such as dry air, at the moment of discharge the 

 resistance along the line of discharge is practically nothing, and 

 therefore all the charge is conducted away. According to 

 Faraday, " the ultimate effect is exactly as if a metallic wire had 

 been put into the place of the discharging particles" ("Re- 

 searches," .series xii.). Most of those tried failed. 



The survival of the fittest has been exemplified in the " plate " 

 protector. In this form — one of the earliest introduced — one 

 thick plate of brass is in connection with the earth, and another 

 similar plate in connection with the line, is placed above it, 

 but separated from it by paper, or by insulating washers. The 

 lightning entering the wire bursts across the paper or air-space 

 in preference to passing through the apparatus, and thus escapes 

 to earth. 



An important modification of this plate discharger has been 

 made by Dr. Werner Siemens, who, by serrating, or grooving 

 with a pointed tool the opposing faces of the two jilates at right 

 angles to each other, converted them into a conductor which 

 was supposed to be one composed of an infinite number of 

 oppc sing points. The remarkalJe action of points in facilitating 

 discharge is well known, and their introduction into lightning 



protectors occurred very early in the annals of telegraphy by Mr. 

 C. V. Walker, F.R.S. 



Messrs. Siemens's arrangement, very pretty in theory, never 

 carried conviction of its value to the mind of the author, because 

 protectors so prepared never singled themselves out as evidently 

 superior to others that were not so prepared, and while the inter- 

 section of the grooves certainly formed mathematical points, they 

 did not form physical or mechanical points, and it is upon the 

 action of this latter kind of points that such remarkable electrical 

 effects are produced. 



Dr. Warren de la Rue having' very kindly placed his well- 

 known battery of 11,000 cells at the disposal of the writer, he 

 prepared four plate-protectors identical in dimensions, excepting 

 that two were serrated, and two were not. The two plates were 

 separated from each other by narrow ebonite washers '01 inch 

 thick. The upper plate was placed in connection with the 

 positive pole, and the lower plate with the negative pole. The 

 number of cells were increased until a continuous current of 

 electricity flowed. 



I. — Plain Plates 



Q 11 Effects produced. 



1,000 ... Slight sparks commencing on completing circuit 



1,080 ... Sparks evident. 



1,200 ... Sparks frequent and abundant. 



1,500 ... Continuous arc. 



2. — Serrated Plates 

 '^Te'llJ."* Effect produced. 



1,000 ... Sparks just commencing on making contact. 



1,080 ... Sparks evident. 



1,200 ... Sparks frequent. 



1,500 ... Continuous arc, but fitful. 



2,000 cells in each produced a continuous stream of electricity- 

 The effect with 1,500 cells was decidedly more marked with 

 the plain plates than with those serrated. The experiments were 

 extremely pretty, and very decided in their character. Hence it 

 appears that grooving is not only of no use, but that it rather 

 deteriorates the value of the protector. 



These experiments confirm very decidedly the accuracy of the 

 figures obtained by Dr. Warren de la Rue and Mr. Miiller on 

 the striking distance between two flat disks given by them in 

 their paper read before the Royal Society (Phil, Trans., vol. 

 169), where it was shown that 1,200 cells struck across -012 inch. 

 Here 1,000 struck across "oi inch, which agrees perfectly with 

 the curve produced by those observers. 



It is the practice in the Post OflSce telegraph department to 

 keep these plates apart by thin paraffined paper '002 inch thick, 

 so that the air surface is really much thinner than that experi- 

 mented upon, and the striking difference of potential only 250 

 volts. 



Messrs. De la Rue and Miiller have shown that for points and 

 various kinds of surfaces opposed to each other, plane surfaces act 

 the best for potentials less than I, SCO volts, and that points are only 

 efficient for high potentials. Now as it is doubtful whether 

 atmospheric electricity causes much higher potential than 1,000 

 volts, it is clear than plane surfaces are the most effective for 

 protecting apparatus. It is quite certain that such plates, plain 

 and smooth, separated by an air space '002 inch thick, will form 

 very efficient lightning protectors. 



The author is very much indebted to Dr. Warren De la Rue 

 for the performance of the experiments in his laboratory. 



Experiments made to determine the Friction of Water upon IVaier 

 at Lo-cu Velocities, by Rev. S. Haughton, M.D., D.C.L. — The 

 author's intention, in commencing the experiments, was to ascer- 

 tain the co-efficient of tidal friction, .and also to ascertain the 

 elevation of water at the equator or pole, necessary to cause a 

 current ; and both these results he hopes to secure with some 

 approach to accuracy. 



The experiments were conducted by means of a spherical ball 

 of granite, unpolished, which was suspended by a pianoforte 

 wire, and allowed to hang freely ; from the brass collar by which 

 the ball was suspended an index projected on each side, the 

 pointed ends of the indices traversing a graduated horizontal 

 circle, whose centre corresponded with the line of suspension. 

 The suspended ball was immersed in water contained in an iron 

 tube. 



On the Tension of Vapours near Curved Surfaces of that 

 Liquids, by G. F. Fitzgerald.— The paper is intended to give a 

 physical explanation of the fact that the tension of a vapour In 



