April 22, 1909J 



NA TURE 



217 



Essays and Addresses. By the late J. H. Bridges. 



With an introduction by Frederic Harrison. Pp. xxi 



+ 307. (London : Chapman and Hall, Ltd., 1907.) 



Price 12s. 6d. net. 

 The essays included in this volume (unobtrusively 

 edited by Prof. L. T. Hobhouse) form an admirable 

 memorial of one of the noblest spirits that have been 

 touched to fine issues by the " religion of humanity." 

 It is, naturally enough, chiefly as a splendid evan- 

 gelist of the Positivist movement that DV. Bridges 

 is considered in the introduction — itself an interest- 

 ing and illuminative essay — which Mr. Frederic 

 Harrison has contributed to the book. But there 

 is no reader, however unsympathetic with the Com- 

 tist propaganda, who could rise from the perusal of 

 these essays without having acquired deep admira- 

 tion for the earnestness and spiritual charm, the 

 learning, worn lightly as a flower, and presented 

 with extraordinary vividness and freshness, the 

 wonderful industry, fecundity, and versatility of the 

 man whose literary achievements were the fruit of 

 the leisure hours of a busy physician and hard-worked 

 Government inspector. 



The scientific reader who first made Dr. Bridges' 

 acquaintance as the learned and indefatigable editor 

 of Roger Bacon will accept almost as a matter of 

 course the masterly summary of his long studv of 

 the great Franciscan, delivered as a university ex- 

 tension lecture in iqo3. He will find in the oration 

 on " Harvey and his Successors " merely another 

 delightful example of the combination of critical, 

 historical, and expository powers that illuminated so 

 effectively the "Opus Majus." He will be prepared 

 also for the familiar knowledge of the mediaeval world 

 shown in the two essays on Dante. But in these 

 latter essays, particularly in the one entitled " Love 

 the Principle," he will have revelation of spiritual 

 powers perhaps unsuspected and of the noblest tvpe. 

 Moreover, his progress through the book will con- 

 stantly deepen the impression that, even more ad- 

 mirable than the ability, the industry, and the taste 

 that made Dr. Bridges so interesting and instructive 

 a critic of topics ranging from Thales to Calderon 

 and Diderot, was the self-sacrificing enthusiasm ever 

 burning at the core of his indefatigable life. 



LETTERS TO THE EDITOR. 

 [The Editoi does not hold himself responsible for opinions 

 expressed by his correspondents. Neither can lie 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. 



With reference to Dr. Cliree's letter published in 

 Nature of April i, may I state that the conditions I postu- 

 lated were violated in every one of the cases quoted in his 

 former letter? The ascents were all made a/(er sunrise and 

 before sunset. They cannot, therefore, be fairly used as 

 evidence to suggest that my conclusions were inaccurate 

 in the direction of underestimating the influence of radia- 

 tion. At the same time, it may be worth while to consider 

 these cases. 



Dr. Chree specified three occasions on which the differ- 

 ences between the temperatures recorded by two instru- 

 ments of different types exceeded 2° C, the maximum 

 differences being- 2°-i, 2°-5, 2°-9 C. The first, as he 

 stated, was probably due to a scale or zero error, one 

 thermometer being continually below the other. The 

 third, on which he lays most stress, occurred in an ascent 

 at Uccle on February 7, 1007. The readings of both 

 instruments agreed during the ascent, the greatest differ- 

 ence being o°-8 C. (or i°i C. at the highest point). 

 During the descent larger differences occurred. Now, at 

 the time of observation, the sky was covered bv a veil of 

 cirro-stratus, and it appears extremely probable that the 

 NO. 2060, VOL. 80] 



instruments, in descending from cold to warmer, perhaps 

 saturated, air, would be affected by condensation of ice- 

 vapour. The difference in exposure and type, combined 

 with the bad conductivity of hoar-frost, may quite reason- 

 ably account for the differences between the temperatures 

 of the two instruments which arose when they left the 

 isothermal region. In addition to this, at the time of 

 maximum difference the downward velocity was about 

 10 m.p.s., and there would be some lag in the instru- 

 ments. This descent was, in fact, exceptional. 



In the second case, an ascent at Strassburg on the same 

 day, the readings indicate a slight lag in one instrument 

 until the lowest temperature is reached. The sudden 

 passage to a relatively warm upper layer was accompanied 

 by a sudden jump of i°-2 C. in the difference between the 

 readings of the two instruments. The type of instrument 

 which shows the lower temperature is the same as that 

 which showed the higher temperature in the Uccle descent. 

 This is just what we should expect if the instruments 

 passed from a saturated layer, in which they became 

 covered with hoar-frost, to a drier region. There is no 

 record of the upper clouds at Strassburg at the time of 

 the ascent, but it occurred simultaneously with the Uccle 

 ascent, so that the explanation is a possible one. 



In an earlier letter Dr. Chree suggested the possibility 

 of errors of + 10° F. in the instrumental records. In 

 order to show as fairly and clearly as possible the errors 

 that may arise, I have taken, for Munich, all the cases 

 from January, 1907, to March, 1908, in which the read- 

 ings from two types of instrument were obtained, and the 

 following table gives the height of the ascent, the extreme 

 differences that occurred, and the mean of the absolute 

 values of the differences at all the points for which they 

 are published. The types of instrument were the same as 

 those considered by Dr. Chree. 



10-5 

 los 

 98 

 14-8 



1 1 o 



1 2 4 



'35 

 127 

 lyo 

 I TO 

 '30 

 129 

 14-2 

 13-4 

 148 

 160 



I'l 



07 

 0-5 

 0-5 



o'3 



0-2 

 0-2 



OS 



°i 



II 



09 



OS 

 o'S 

 0-5 

 0-8 



0-8 



1-5 ... -I-.? 

 14 ... -2-8 

 17 ... -29 



In interpreting these results, it ought to be borne in 

 mind that they are chiefly from ascents, and include errors 

 owing to lag, which could be largely eliminated in deal- 

 ing with the observations. The records I have seen usually 

 show that the thermometer, which is higher in the ascent, 

 is lower during the descent, and that the lag occurs almost 

 entirely in the worse instrument, so that the differences 

 are representative of the absolute errors arising from this 

 cause. Considering the very many sources of error to be 

 guarded against, especially the difficulty of testing the 

 instruments at very low temperatures under the conditions 

 to which they are to be exposed, I can only regard these 

 results as a tribute to the care and ingenuity displayed 

 by those engaged in the experimental exploration of the 

 upper air. 



Dr. Chree does undoubted service in directing attention 

 to the need for great care in testing and comparing instru- 

 ments, but I think he is inclined to be a little unjust to 

 those who are tackling the difficulties of upper-air investi- 

 gation and nomenclature. These difficulties are exemplified 

 by the examples he quoted and by a term which be him- 

 self accepts, apparently without demur, when he describes 

 a phenomenon as an " inversion of temperature." 



Personally, I am quite prepared to discard the term 

 " isothermal " when another is suggested which is short, 

 equally expressive, more accurate, and more characteristic. 

 The greatest variation of temperature in a vertical direc- 



