304 



NATURE 



[July 28, 1898 



OUR ASTRONOMICAL COLUMN. 



Astronomical Occurrences in August:— 

 August 8. loh. 7m. to loh. 44m, Occultation of ^ Arieds 

 (mag. 5 '8) by the moon. 

 8. Saturn. Outer minor axis of outer ring, 17" •37. 

 8. I5h. Mercury at greatest elongation (27° 22' E.). 

 ID. Meteoric shower from Perseus (radiant 45° + 57°)- 



11. I4h. 49m to I5h. 38m. Occultation of liSTauri 



(mag. 5 '4) by the moon. 



12. I2h. 31m. to I3h. 19m. Occultation of 8 Gemi- 



norum (mag. 6*5) by the moon. 



15. Venus. Illuminated portion - 0655, diameter = 

 1 7" -4. 



15. Mars. Illuminated portion = 0*889, diameter = 

 6";o. 



15. Jupiter. Polar diameter = 29" 8. 



15. Saturn. „ ,, = 1 5" '8. 



19. 6h. Venus in conjunction with Jupiter (Venus 

 i°Si'S.). 



23. 9h. 32m. Minimum of Algol (;8 Persei). 



25. loh. Mars in conjunction with Neptune (Mars 1° 

 13'N.). 



28. I2h. 53m, to I3h. 43m. Occultation of a Capri- 

 corni (mag. 5 "6) by the moon. 

 The Minor Planets. — Mr. John K. Rees, in a lecture 

 before the New York Academy of Sciences {School of Mines 

 Quarterly, vol. xix. No. 3), delivered a very interesting discourse 

 on the history of the discovery of the minor planets, a reprint of 

 which has been sent to us. Mr. Rees describes from the begin- 

 ning how, after the discovery of Uranus by Herschel, Prof. 

 Titius, of Wittenberg, pointed out the existence of a remark- 

 able symmetry in the disposition of the bodies constituting the 

 solar system. It was he who suggested the relationship now 

 known as " Bode's law," Prof. Bode putting into the place of 

 the missing body a hypothetical planet. It is not generally 

 known, perhaps, that Von Zach in 1785 actually calculated 

 elements for this "unseen and unfelt body," and for fifteen years 

 kept in his mind the need for a careful search. At the beginning 

 of this century he organised, what was termed jocularly by him 

 the " Celestial Police " to track and intercept this fugitive 

 object, a force for the express purpose of systematically scanning 

 the heavens ; but it was left for the astronomer, Piazzi, who 

 found the first of what eventually proved a series of small bodies, 

 although he was carefully observing the heavens for quite 

 another purpose, namely the formation of a star catalogue. This 

 discovery of the minor planet Ceres was the first of many 

 which followed, and the introduction of photography in this 

 branch of observation has brought to light many small bodies 

 which are now numbered in hundreds, besides rendering the 

 task, which was beset with great difficulties, one that is now 

 simplicity itself. 



The Moon and Auror/e.— From the earliest times the 

 presence ofaurorse was in some way connected with the influence 

 of the moon, and there may be some, even to-day, who are 

 inclined to hold to this opinion. Prof. H. A. Hazen, in the 

 Monthly Weather Review (vol. xxvi. No. 161), discusses the 

 evidence of such supposed influence, using as his data the 

 observations made by the regular observers in the United States 

 Signal Service. We need not, however, refer to the curves and 

 tables which are brought together by Prof. Hazen, but simply 

 quote the words which he uses in summing up the whole of the 

 investigation in question. He says: "It will be seen readily 

 that the whole theory of a lunar influence upon auroras breaks 

 down from first to last under this analysis." That the appear- 

 ances of aurorte may be connected with the periodicity of sun- 

 spots is another matter, and it is here that probably a close 

 connection exists. 



Apropos of aurorse, we notice that Prof. Cleveland Abbe is 

 publishing a very detailed and important historical account of 

 the altitude of aurorse above the earth's surface as determined 

 • by observers all over the world. The first of these articles 

 appears in Terrestrial Magnetism (vol. iii. No. 2), and is well 

 worth reading by those who are interested in this important 

 question. 



Mars in 1896-7.— Prof. V. Cerulli has just published, in the 

 Pubblicazioni deW Osservatorio private di Collurania (Teramo) 

 (No. i), a most important memoir of the planet Mars, as 

 observed by him during the period 1896-7. The volume covers 



NO. 1500, VOL. 58] 



no less than 126 pages, and is accompanied by numerous plates, 

 forming a valuable addition to our knowledge of this interesting 

 planeL Perhaps a special feature of this publication is the 

 determination of the latitudes and longitudes of sixty of the 

 most prominent markings on the surface ; and this will, without 

 doubt, be found most valuable to those who wish to locate 

 accurately any surface features which they may from time to 

 time observe. In the remaining portion of the work Prof. 

 Cerulli discusses these and other surface markings which were 

 seen during this period of observation, and a comparison of 

 these with the observations of others should be found of great 

 interest. 



RECENT WORK IN THERMOMETRY. 

 'T'HERMOMETRY is one of those departments of physics 

 -*■ which are left almost exclusively to specialists, and writings 

 on the subject are apt to assume an amount of preliminary know- 

 ledge not possessed by physicists in general. There thus appears 

 room for a brief account in popular language of recent progress. 

 The space at my disposal being limited, I am obliged to confine 

 my remarks to a comparatively small number of researches, ant) 

 I can hardly hope that my choice of matter will meet with un- 

 mixed approval. 



Thermometry possesses two main branches, which, though 

 intimately connected, are yet more or less distinct. One 

 branch deals with the detection of extremely minute differ- 

 ences of temperature, or the subdivision of small temperature 

 intervals ; the other aims at assigning a definite numerical 

 value to temperatures on an exact scale. A worker in the 

 first department may employ apparatus showing differences 

 of one-millionth of a degree Centigrade, and he may even 

 believe that he is measuring temperature to this degree of 

 nicety. A worker in the second department, unless endowed 

 with an exceptionally optimistic temperament, will probably 

 not profess to measure temperature to nearer than the one- 

 thousandth of a degree, and that only between the freezing 

 and boiling points of water. Here I shall consider almost 

 exclusively the question of the determination of temperature 

 in absolute measure. 



The first requisite is a normal scale to which all measure- 

 ments can be referred. An ideal scale should be perfect ir> 

 theory, and easily and exactly realisable in practice. From 

 the former point of view. Lord Kelvin's absolute thermo- 

 dynamic scale is generally regarded as facile princeps. In 

 the meantime, however, it fails to satisfy the second con- 

 dition. The International Committee of Weights and Mea- 

 sures, representing all the leading Powers, including Great 

 Britain, accordingly selected in 1887 for the normal scale 

 that of the hydrogen constant-volume thermometer, the gas 

 when at 0° C. to be under the pressure of i metre of mer- 

 cury under standard conditions ; on this scale equal incre- 

 ments of temperature answer to equal increments of pressure. 

 Apparently the choice was due mainly to two considerations, 

 viz. the very low freezing point of hydrogen, and the exist- 

 ence of theoretical and experimental grounds for believing its 

 scale to approach Lord Kelvin's absolute scale more nearly 

 than that of any other common gas. Whether hydrogen will 

 prove a manageable substance at high temperatures seems 

 open to some doubt. Failure in this respect would be a 

 serious drawback, in view of the rapidly increasing import- 

 ance of high temperature measurements. 



After the choice of a normal scale, we are next concerned 

 with its relationships to other scales that are, or have been 

 previously, in use. Here, however, one difficulty is conspicu- 

 ously present. Nothing is commoner than such a statement 

 as that a certain temperature was observed on the scale of 

 the air thermometer ; but there are air thermometers and air 

 thermometers. Quite apart from the distinction between con- 

 stant volume and constant pressure instriiments, there are 

 questions as to the pressure at 0° C, the purity of the air, 

 the sufficiency or insufficiency of the corrections applied to 

 the observed readings, and a host of others. In most invest- 

 igations thermometry is but a means to an end, and ob- 

 servers are apt to treat somewhat lightly of preliminaries 

 which are not of general interest. On the other hand, an 

 observer is very apt to attach undue significance to the agree- 

 ment between the several observations he makes, overlooking 

 the fact that in thermometry such agreement need imply no 



