January 7, IC09J 



NA TURE 



;8i 



none of llio tlirft- is dead-beat. Thus the apparent ampli- 

 tudes of tlie records will depend on the proximity of the 

 natural period to that of the seismic wave. This may 

 account for the fact that whilst the seismograph and the 

 horizonlal-forcc magnet indicate a maximum of disturb- 

 ance at from 4.31 to 4.33, the declination magnet indicated 

 more disturbance at 4.36. A movement of 17 mm. on the 

 seismic trace answers to a lilt of fully 9", but it may be 

 produced in a variety of ways, and no immediate deduc- 

 tion is possible as to either the character or the amplitude 

 of the disturbing motion. C. CiiKi;!;. 



January 1. 



[.\dded January 4. — -An examination of the glass scale 

 used with the magnetic curves shows that a correction 

 of about —0-5 minute is required to the times deduced by 

 it. This brings the above times from the magnetic and 

 seismic curves into even more perfect agreement.] 



The Commercial Products of India. 



It is not customary for an author to reply to his 

 reviewers, but I trust you w-ill permit me to depart from 

 that usage. Captain A. T. Gage, superintendent of the 

 Royal Botanic (iurdens, Calcutta, stands, to my recent 

 work " The Commercial Products of India," in an entirely 

 different position from an anonymous reviewer, and his 

 opinion, as expressed in certain passages of the notice 

 published in Nature of December 17, 1908, therefore seems 

 to me to call for a reply. 



Captain Gage accuses me of having " unnecessarily spun 

 out " certain articles by a " failure to discriminate between 

 essential and superfluous information and between proved 

 facts and mere opinions not worth recording." he then 

 proceeds to exemplify that contention by quoting one 

 sentence regarding tea. Removed from its context, that 

 particular passage might fall under the condemnation he 

 has passed upon it, but when read in connection with the 

 sentences immediately preceding and following, its mean- 

 ing and value arc, I venture to think, abundantly brought 

 out. The contention, it will be seen, is advanced that 

 even in China tea appears to have been first used as u 

 vegetabli' or medicine, and that it was not until the fourth 

 century that its modern usage as a beverage began to 

 attract attention. If I am justified in assuming that many 

 of my readers may find interesting what had proved such 

 to myself, it seems likely that the fact that the habit of 

 tea drinking is not very ancient, even in China, will not 

 be regarded as superfluous information. 



Then, again. Captain Gage apparently objects to my 

 method of exemplifying the failure, so far, with rhea 

 cultivation in Kangra. I have given prominence (so he 

 aftirms) to the fascinating effect on myself personally of 

 the undying faith of a very old lady. Now anyone at all 

 familiar with the recurrent interest in rhea and China 

 grass — aware, In fact, of the extent of capital eyen now 

 at stake — would hesitate to pronounce rhea, as Captain 

 Gage has done, "a distinctly , doubtful crop." The fibre, 

 at all events, is in itself immensely valuable, hence, in 

 reviewing India's position in the controversy of future 

 production, I felt myself compelled to give actual results 

 in preference to . dogmatic pronouncements. My posi- 

 tion regarding India's future participation is brieliy that, 

 while we have the " undying faith " of some of the 

 pioneers, the results so far attained have not been exactly 

 favourable ; but I have urged that there is distinctly a 

 future for the crop when certain misleading statements 

 and misconceptions have been elTectively removed. In 

 other words, I by no means concur with Captain Gage 

 that rhe.T is " distinctly a doubtful crop." 



But my reviewer has fallen foul of me because my 

 abridged articles on tea and rhea (as he thinks) are longer 

 than the originals. Perhaps I may be permitted to ex- 

 plain that the chief difficulty I experienced in writing the 

 work in question wms the necessity, imposed on me, to 

 restrict and restrain my efforts on every hand by calcula- 

 tions or ratios of sp.ace to articles, and by the final 

 .nccomplishment of the entire task within one volume. 

 Captain Gage's criticisms on the science of circumscrip- 



XO. 2045. VOL. 79] 



tion arc, in fact, examples of thai very difficulty, only 

 that he fails in the all-important detail of accuracy. If 

 he will consult again the original work he will perhaps dis- 

 cover that it often happened that a subject was there dealt 

 with under two or more positions. In the new work e.ich 

 had to be disposed of once and for all. Hence Boclnncria 

 nivea — Rhea — does not have fifteen pages in the old and 

 si.xteen pages in the new work, as Captain Gage afiirms, 

 but sixty and sixteen pages respectively. So also Camellia 

 thea — Tea — does not have fourteen pages in the old and 

 thirty-five pages in the new work, as Capt.'iin Gage also 

 affirms, but eighty-two and thirty-five respectively. The 

 articles on these two subjects thus occupy, as ne.ar as 

 possible, the exact spaces reserved for them in the scheme 

 of the new publication. George Watt. 



Richmond, December 19, 1908. 



The Isothermal Layer of the Atmosphere. 



I HAVE read with much interest the letters on this sub- 

 ject that appeared in Nature during last February and 

 .March, and ,also, the account of the discussion at 

 the British .Association (Nature, October i, 190S), and 

 my only e.xcuse for re-opening the question at this late 

 date is that a point seems to have been overlooked which 

 appears capable of explaining tlie phenomena without any 

 appeal to an isothermal layer. Both in the correspondence 

 and in the discussion several physicists cast doubt on the 

 accuracy of the thermograms, but, so far as I have seen, 

 only Mr. A. L. Rotch, at the British Association, men- 

 tioned that his instruments were verified for low tempera- 

 tures and pressures. The following physical effect on the 

 barographs does not appear to have been mentioned, and 

 I should be glad to know what precautions are taken to 

 eliminate it in practice. Pressures are necessarily regis- 

 tered by aneroids, and it appears to be assumed throughout 

 all these discussions that a lower pressure on an aneroid 

 means a higher altitude, but this is not so. In 1892, when 

 I was a temporary observer in Ben Nevis Observatory, Mr. 

 Edward Whympcr visited the district to have some four- 

 teen or fifteen aneroids of various sizes compared with the 

 mercurial barometers at the low-level station, and as soon 

 as possible afterwards at the top of the hill. It was in- 

 variably found that the indexes kept on falling after the 

 aneroids had been brought to rest in the observatory. The 

 rate of fall was at first fast, but became slower as time 

 went on, and it depended upon the difference of pressures 

 between the two stations and also upon the time taken in 

 transit from one to the other, being greater for greater 

 differences of pressure and less for longer times of transit. 

 The aneroid would tend to give the true pressure 

 immediately on arrival or after some hours, according as 

 the standardisation had been rapid or slow. The effect is 

 due to a kind of elastic fatigue, and was reversed on 

 returning the aneroids to sea-level. 



Mr. A. Mallock, F.R.S. (Proc. Roy. Soc, vol. Ixxx., 

 p. 530), has shown that up to the altitudes corresponding 

 to pressures of about 100 millimetres of mercury the 

 velocity of the balloons increases slightly, but at these 

 altitudes it decreases so suddenly that the hypothetical 

 balloons with which he deals must there have ceased rising. 

 It is clear that at such altitudes the conditions are most 

 favourable to the operation of elastic fatigue. The change 

 of pressure to which the aneroid has been subjected is 

 considerable ; the time of ascent is fairly rapid, and the 

 velocity is suddenly destroyed ; but although the balloon 

 may cease to rise, the apparent pressure docs not cease 

 to fall. Consequentlv, when the barogram is deciphered, 

 if the effect of elastic fatigue is ignored, an increased 

 height will be inferred at the same epoch as a constant 

 temperature. There mav even be an increase of tempera- 

 ture if the balloon should leak slightly or if the gas should 

 be sluggish in acquiring the low temperature of the air 

 into which it has risen, and, cooling somewhat, causes 

 the balloon to descend slightly. It should be remembered 

 that at these altitudes a small change of pressure corre- 

 snonds to a very consider'-hle change of altitude, so that 

 this effect of fatigue would be greatly exaggerated. The 

 great differences of altitude at which the isothertivnl l.iyer 



