January 21, 1909] 



NA TURE 



541 



temperature of the air with which it is in contact. It is 

 hard to suggest a better arrangement for getting some 

 notion of the low temperatures which the upper air appears 

 to delight in producing and maintaining in the most trying 

 circumstances ; nevertheless, it is easy to show that the 

 temperature of the instrument will differ from that of 

 the air by a variable interval depending on the speed of 

 the balloon, the density of the air, and the intensity of 

 the radiation from earth, clouds, balloon, sun, air, and 

 vapour. 



.Suppose I wish to take the temperature of the feed air 

 below the fire-bars of a furnace. If the current of air 

 were sufticient, and the screening of the thermometer 

 almost perfect, I might get a close approximation, but if 

 the draught relaxed in speed or density, or if the screen- 

 ing became less perfect, the thermometer would respond 

 to the radiation by which it was surrounded and rise above 

 the temperature of the air. A ballon sonde is such an 

 apparatus. It is in a warm situation, but is surrounded 

 by an intensely cold medium. It is a speck extremely 

 close to a great warm planet, and bathed in his radiations 

 and reflections — to say nothing of sun-rays, which some- 

 limes complicate the problem. The screen is open to the 

 earth below and to the balloon above, and the instrument, 

 though screened itself, follows in the wake of the un- 

 screened balloon and is fanned by the air that has passed 

 over its heated surface. 



In the ascent the thermometer reading falls briskly, and 

 soon reaches a figure which may be ioo° or more below 

 what it would stand at if it were screened from air currents 

 for a few minutes ; but this gap between the temperature 

 of the instrument and the temperature proper to its posi- 

 tion cannot be extended indefinitely. The up-rush of the 

 balloon attains a maximum velocity and declines, and the 

 density of the air also rapidly diminishes. When the 

 receipt by radiation equals the loss by conduction, the 

 thermometer has reached its minimum, and enters the 

 so-called "isothermal layer," the regularity of the occur- 

 rence of which on the traces is due to the similarity of 

 pattern of balloon and outfit and of the other circum- 

 stances. I know that Mr. Dines contends that speed 

 upwards or downwards can have no effect on the thermo- 

 meter, but he takes no account of the circumstance that 

 heat is constantly entering the instrument, and that it is 

 solely the current of air that keeps down its temperature 

 by removing the intruding heat. 



It seems a pity that the following question stands 

 barred : — Whv is the material Air so cold where the 

 material balloon and other instruments would he so 

 warm? R. F. Hughes. 



16 Westmoreland Street, Marylebone, W., January 9. 



If all balloon ascents had been made by day, I confess 

 that I should be inclined to agree with Mr. Hughes and 

 think that the recorded temperatures were due to radia- 

 tion, but that idea is disposed of, to my mind, by the fact 

 that the isothermal column of air shows just as plainly in 

 ascents made after sunset as in those made in the day. 

 .\t night the thermograph must receive some heat b>' 

 radiation from the earth, and lose some by radiation into 

 space, but both amounts must be infinitesimal in com- 

 parison with that which would be given to it by the sun. 

 If, then, exposure to the sun does not seriously alter the 

 temperature, and it does not do so even at the greatest 

 height provided there is a moderate amount of vertical 

 motion, the effect of the radiation after sunset must be 

 utterly insignificant. That solar radiation in the ordinary 

 conditions is not important is proved by the fact that if 

 the balloon bursts, and therefore does not float, it is not 

 possible to sav from the trace alone if the ascent was by 

 night or by day. There have been cases in which the 

 balloon did not burst, and the temperature at the top reached 

 the freezing point of water. If I asserted that the rate of 

 ascent does not matter, I must plead guilty to a mistake, 

 hut I think I said " apart from radiation," and I still 

 believe that radiation at night to and from the bright metal 

 of the thermograph is so trifling that the rate of ascent is 

 of no consequence. There is also the fact that the up- 

 trace, where the motion is comparatively slow, is identical 

 with the down-trace where the motion is moid. 



W. H. Dines. 



An Electromagnetic Ptoblem. 



Since the solution of the problem put forward by Prof 

 Comstock in Nature of November 19, 1908, is apparently 

 not obvious to everyone, will you permit me to point out 

 that, so far as I can see, the difficulty arises, not from 

 any peculiarity of the laws of electromagnetism, but from 

 a simple misconception of the meaning of dynamical 

 terms ? 



Prof. Comstock says that when an electrified sphere 

 expands it loses electrostatic energy, but does not gain 

 either kinetic energy (for the sphere has no massj or 

 magnetic energy (for the resultant field due to the motion 

 of all parts of the sphere is everywhere zero). Now the 

 energy of a conservative system, such as is considered, is. 

 measured by the amount of work which it can do on, 

 some e.\ternal system in passing from its original to some- 

 defined final state; the amount of the work which can be 

 done, and therefore the amount of the energy, will vary 

 according to the external system which is chosen, and the 

 principle of the conservation of energy will be true only 

 if the same external system is taken in measuring the 

 amount of work that can be done at various times during 

 the change. 



When he states that the magnetic energy of the expand- 

 ing sphere is zero. Prof. Comstock is taking as his external 

 system, on which work is to be done, a system uncon- 

 nected with and independent of the expanding sphere ; but 

 the electrostatic energy of the sphere with respect to such 

 a system is quite unaltered by the e.xpansion, if the system 

 is either wholly within or wholly without the sphere 

 throughout the expansion, and the change in the electro- 

 static energy which ensues, if any part of the system passes 

 through the surface of the sphere during the expansion, is 

 independent of the discreteness or continuity of the electrifi- 

 cation on the sphere, and perfectly consistent with the 

 conservation of energy. Adopting such an independent 

 system as that on which work is to be done, there is no. 

 relevant change in either the electrostatic or the magnetic 

 energy. 



On the other hand, when he says that the sphere in 

 expanding loses electrostatic energy. Prof. Comstock is 

 taking as the system on which work is to be done part 

 of the expanding sphere or some system connected rigidly 

 therewith ; but then it is clear that in estimating the 

 magnetic energy no account must be taken of the magnetic 

 field due to the motion of this part. Leaving out of account 

 the magnetic field due to this part of the sphere, a simple 

 calculation shows that the inagnetic field due to the motion 

 of the rest of the sphere relatively to this part is'.not zero 

 everywhere, but that the value of ^H'/Stt, integrated 

 throughout the entire field, is equal to the value of the 

 electrostatic energy with reference to this part lost in 

 expansion. Norman R. CAMrBEi.L, 



Trinity College, Cambridge, January 16. 



Radium in the Eartb. 



In the discourse entitled " Radio-active Changes in th^ 

 Earth," delivered at the Royal Institution by the Hon. 

 R. J. Strutt, and printed in Nature of December 17, 1908, 

 the lecturer advanced the opinion that the mineral beryl 

 contained an hitherto unknown element from which the 

 (omparatively large quantity of helium present is generated. 



This interesting and remarkable conclusion has induced 

 me to direct attention to a statement which occurs in ' a 

 paper entitled " The Heat of Formation of Glucinum- 

 Chloride," by J. H. Pollok (Chem. Soc. Trans., 1904,. 

 p. 603). Mr'. Pollok prepared a large quantity of basic 

 glucinum carbonate from 2000 grams of beryl, and during 

 the preparation of this compound he detected the presence 

 of another substance, the nature of which he was not able 

 to ascertain. His statement is as follows ; — " This pre- 

 cipitate consisted chiefly of iron, zinc, and nickel sulphides, 

 but another substance appeared to be present ; its amount 

 was, however, too minute to admit of any satisfactory 

 conclusion being drawn regarding it. This sulphide has 

 also been observed by Kruss and Moraht." 



Percy Edgerton. 



The I^aboratories, 61 Cornhill, London, E.C., 

 December 31, 190S. 



NO. 2047, VOL. 79] 



