Afay 13, 1880] 



NATURE 



Z3 



win his spurs. The lowest depth to which he has 

 descended is 25 feet. He has walked under water a 

 distance of four hundred yards in a straight line. 



Some improvements may be made in the arrangements. 

 He might be supplied with a feeding-apparatus, and so 

 remain under water several hours longer than he has 

 done. At present he finds from two to three hours no 

 difficulty. 



The experiments I have made with the apparatus 

 indicate that the dress and apparatus may be used 

 for entering wells, burning houses, and mines that are 

 charged with suff"ocating gases. In the mine the dress 

 would be mvaluable, and if a telephonic connection could 

 be set up between the man in the dress and the outside 

 world— an adaptation I believe to be quite possible— a 

 remarkably useful advance would be made. 



I will now ask Mr. Fleuss to make one experiment 

 which will be a visible exposition of the perfection of his 

 apparatus as he stands equipped in it. The directors of 

 the Royal Institution have been so good as to lend me 

 the glass chamber in which Prof. Tyndall experimented 

 when he was demonstrating the mask he invented for 

 breathing in an atmosphere charged with dense fumes of 

 smoke. This chamber I have had charged with carbonic 

 acid, so that it has in it an irrespirable atmosphere. In it, 

 as you will see, a candle cannot be hghted, and a taper 

 will be extinguished. Mr. Fleuss will go into the cham- 

 ber, sit down in it, and wait there until the current of 

 carbonic acid which is being admitted forms an absolute 

 atmosphere of the gas to above the level of the top of 

 bis helmet, and there he will remain, if we like, until the 

 supply of oxygen in the helmet is exhausted. 



The next step onward will be to construct a small 

 ■closed canoe, in which the apparatus can be fitted on a 

 larger scale, and in which men, or those who are in the 

 canoe, can rise or sink in the water and be propelled 

 tinder the water. This is a certain extension of the 

 system now under our consideration, and when it is com- 

 pleted, my idea that the next greatest geographical dis- 

 coveries will be made on the floors of the great oceans 

 may not be so far wide of the mark as was once 

 supposed. 



B. W. Richardson 



THE AURORA BOREALIS"- 



OUR experiments on the electric discharge, which have 

 been already published in the Phil. Trans, and the 

 Proceedings of the Royal .Society, enable us to state with 

 some degree of probability the height of the aurora 

 borealis when its display is of maximum brilliancy, and 

 also the height at which this phenomenon could not occur 

 on account of the great tenuity of the atmosphere. 



In Part 111. of our electric researches, Pliil. Trans., 

 Part I. vol. 171, we have shown that the least resistance 

 to the discharge in hydrogen is at a pressure of o'i>\2 

 millim., 845 M ; after this degree of exhaustion has been 

 reached a further reduction of pressure rapidly increases 

 the resistance. When the exhaustion has reached o'ooa 

 millim., 3 M, the discharge only just passes with a poten- 

 tial of 11,000 chloride of silver cells (11,330 volts) ; at the 

 highest exhaust we have been able to obtain (and which 

 we believe has not been surpassed), namely, o'oooo55 

 millim., o'o66 M, not only did 11,000 cells fail to produce 

 a discharge, but even a i-inch spark from an induction- 

 •coil could not do so. 



Although we have not experimentally determined the 

 pressure of least resistance for air, we have ascertained 

 that while the discharge occurs in hydrogen at atmo- 

 spheric pressure between disks o'22 inch distant, they 



» "On the Height cf the Aurora Borealis." Paper read at the Royal 

 Society. By Warren De La Rue, M.A., D.C.L., F.R.S., and Hugj W. 

 MuUer, Ph.D., F.R.S. 



require to be approached to o'i3 inch to allow the dis- 

 charge to, take place in air. We may therefore assume 

 that the pressure of least resistance for air is 



°~^^-J^ '^ = 0-379 millim., 498-6 M. 



At a height of 37-67 miles above the sea level, the 

 atmosphere would have this pressure (neglecting change 

 of temperature), and therefore the display at this elevation 

 would be of maximum brilliancy and would be visible at 

 a distance of 585 miles. 



The greatest exhaust that we have produced, 0-00005; 

 millim., o-o66 M, corresponds to a height of 81-47 miles, 

 and as 11,000 cells failed to produce a discharge in 

 hydrogen at this low pressure, it may be assumed that at 

 this height the discharge would be considerably less 

 brilliant, especially in air, than that at 37-67 miles, the 

 height of maximum brilliancy. 



At a height of 124-15 miles the pressure would be only 

 o-oooooooi millim., o-ooooi M, and it is scarcely probable 

 that an electric discharge would occur with any potential 

 conceivable at such a height. 



The colour of the discharge varies greatly with the 

 tenuity of air or other gas with the same potential. Thus 

 in air at a pressure of 62 millim., 81579 M, the discharge 

 has the carmine tint which is so frequently observed in 

 the display of the aurora ; this corresponds to an altitude 

 12-4 miles, and would be visible at a distance 336 miles. 

 At a pressure of 1-5 millims., 1974 M, corresponding to a 

 height of 30-86 miles, the discharge becomes salmon- 

 coloured, having completely lost the carmine tint. At a 

 pressure of 08 millim., corresponding to 33'96 miles, the 

 tint of the discharge is of a paler salmon colour, and as 

 the exhaust is carried further it becomes a pale milky 

 white. The roseate and salmon-coloured tints are always 

 in the vicinity of the positive source of the electric 

 current, the positive luminosity fades away gradually, and 

 frequently becomes almost invisible at some distance 

 from its source ; as, for instance, in the hydrogen discharge 

 at a pressure of 2-3 millims., 3027 M, shown in the 

 accompanying figure, H, which resembles in some 



respects the phenomena of the aurora. The discharge 

 at the negative terminal in air is always of a v-iolet hue, 

 and this tint in the aurora indicates a proximity to the 

 negative source. 



The following table, with the exception of pressure 



