-,66 



NA TURE 



[January 27, 1910 



number, the name, the maximum magnitude, the co- 

 ordinates for 19000, the precession, and the B.D. 

 number for each star. 



The volume is in quarto form, neatly and strongly 

 bound, and altogether promises the addition of a 

 valuable series to current astronomical publications. 



LETTERS TO THE EDITOR. 



[The Editor does not hold himself responsible for opinions 

 expressed by his correspondents. Neither can he undertake 

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

 manuscripts intended for this or any other part of Nature. 

 No notice is taken of anonymous communications.] 



Upper-air Temperatures Registered Outside and 

 Inside Balloons 



Attention has been directed several times in Nature 

 to the interest attaching to the knowledge of the rate at 

 which the gas inside rubber balloons takes up the tempera- 

 ture of the air outside. In this connection the results of 

 two registering-balloon ascents from Manchester, made at 

 the suggestion of Mr. Gold, are useful as giving some idea 

 of the magnitudes of various possible errors. 



The ascents were made on July 2, 1909, at 6.50 a.m. 

 and 10.17 P-Ti- respectively. Each balloon carried two 

 instruments, of which one was suspended in the usual 

 manner about 4 metres below the balloon, while the other 

 was fixed inside the balloon. The latter instrument was 

 kept approximately in the centre of the balloon by means 

 of rubber stays. The results are given in the following 

 tables : — 



Table I. — (6.50 a.m. Ascent.) 



Temperature °C. 



'J".\ni.E II.— (10.17 p.m.) 



Temperature ■( 



1 1 -5 

 T 2 '5 



- 05 



- 6 



- II 

 -17 

 -23 

 -28 

 -34 

 -40 s 

 -48 

 -55-5 

 -55 5 

 -56 

 -57-5 



+ 6 



- o'S 

 6 



- 1 1 

 -'7 

 -23 

 -31 

 -38 

 -44 

 -50 

 -56 



-57-5 

 -58-5 



57-5 



The traces of the two instruments sent up at 6.50 a.m. 

 emphasise the large possible error arising from insolation 

 should the vertical velocity of the balloon fall below the 



NO. 2100, VOL. 82] 



value necessary for efficient ventilation. The temperatures 

 recorded during the ascent diverge as the height increases, 

 the maximum difference — nearly 40° C. — occurring at the 

 maximum height attained, i.e. 15 km. That the diverg- 

 ence is not due to a systematic difference between the two 

 instruments is shown by their good agreement during the 

 greater part of the descent. In this connection it is of 

 interest to compare the experimental results with the 

 mathematical computations made by Mr. Gold in Nature, 

 March 18, 1909, p. 68. 



Substituting in his equations the values corresponding 

 to the Dines instrument and the small rubber balloons 

 employed in the ascent, and assuming that half the incident 

 solar radiation is absorbed, we find that (T — S), the 

 temperature difference between the gas inside and the air 

 outside, may be 20° C. This is considerably less than the 

 actual value found (30° C), but the discrepancy is easily 

 explained by the fact that the radiation which penetrates 

 the semi-transparent envelope strikes the almost im- 

 ventilated instrument inside. That this accounts partly for 

 the difference is evidenced by the fact that the tempera- 

 ture ceases to fall near n km., where the pressure is 

 about 150 mm., i.e. the pressure below which the natural 

 convection has been found to be ineffective. 



The night ascent gives more definite information. The 

 trace of the outside instrument in this case was defective, 

 and it was assumed that the descent readings of the inside 

 instrument gave a close approximation to the readings of 

 the outside one. The doubling of the trace at low alti- 

 tudes is due to the high initial temperature of the gas 

 inside the balloon — that of the laboratory in which it was 

 filled — and to the effect of daily variation of temperature 

 during the time interval between the beginning of the 

 ascent and the end of the descent. From 2 to 6 km., 

 however, the temperature inside the balloon is the same 

 as that of the outside air. At about 6 km., where there 

 is a considerable increase in the temperature gradient, a 

 lag is developed, which increases to about 8-5 km. where 

 the temperature difference reaches 5° C. Subsequently the 

 lag steadily diminishes, and at 10-5 km. the temperatures 

 are again in agreement to within 1° C. Applying Gold's 

 equation to this case, we find, on substituting the values 

 appropriate to the ascent, that the theoretical lag at 

 II km. is about 07° C, a rather remarkable agreement 

 in consideration of the fact that the equation is admittedly 

 only approximate. It would appear, therefore, that what 

 might be called the " natural " lag of the balloon tempera- 

 ture in night ascents is small, and that the lag indicated 

 between 6 and 1 1 km. arose from special circumstances 

 possibly connected with the humidity of the air. The 

 difference between the ascent and descent traces above 

 II km. may possibly be attributable to the same cause as 

 the divergence from 15 to 10 km. of the descent traces 

 of the two instruments employed earlier in the day. The 

 results demonstrate conclusively the very large effect of 

 solar radiation compared with that of terrestrial radia- 

 tion, and indicate that errors in temperature due to air 

 which has already been in contact with the balloon and 

 to radiation from the balloon are, in night ascents, 

 negligible. 



In the daytime, however, errors arising from these causes 

 may be considerable. This may, in fact, explain why the 

 differences between the upward and downward traces in 

 ascents by day are more frequent and of greater magni- 

 tude than those found for night ascents. 



W. A. Harwood. 



Physical Laboratory, University, Manchester. 



Avogadro's Hypothesis. 



I SHOULD like to direct the attention of Mr. Woolhouse 

 (Nature, January 20) to the little book " .Avogadro and 

 Dalton, the Standing in Chemistry of their Hypotheses " 

 (Edinburgh : W. F. Clay, 1904), in which Dr. Meldrum 

 discusses with great force and discrimination the exact 

 position which should be given to Avogadro's hypothesis. 

 Dr. MeldruiTi also deals faithfully with those who have 

 made light use of the word " law." 



.'\. Smitiiells. 



The University, Leeds, January 24. 



