494 



NATURE 



LMakch 23, 1899 



OUR ASTRONOMICAL COLUMN. 



CoMEr 1899 a (Swii-T). — The following positions of the 

 comet are taken from a circular received from the Centralstelle 

 at Kiel :- 



Ephemeris for 1 2/; . Berlin M. T. 



As will be seen from the table, the Virightness is rapidly in- 

 creasing as perihelion is approached ; the possibility of observing 

 the comet will depend on the local conditions of the sky near 

 the horizon. It sets now about an hour after sunset. As a 



guide for its recognition the accompanying chart of the region is 

 given, which shows the path of the comet from the time of its 

 discovery. 



Tuttle's Comet. — Mr. J. Rahts continues his ephemeris of 

 this comet in Asl. Nach., Bd. 14S, No. 3552. He also gives 

 the elements for the epoch 1899 May 140 Herlin mean time. 



M = 359 59 467 



» = 116 29 3 01 



fl. = 269 49 53-6 .- 19000 



i = 54 29 t6 3| 



■C = 55 15 237 

 p. = 2S9"-6234 

 log « = 0757 10S5 



It may be interesting to many to recall the past history of 

 this comet It was discovered by Tultle at Cambridge, U.S..\. , 

 on January 4, 1858 (--/.(/. jVaiA., No. 1125). Some time after 

 it was recognised to be identical with the comet 1790 II., and 

 its period determined to be about i j 7 years. Confirmation of 

 this was provided by its return in 1S71 and again in 1885, pass- 

 ing perihelion in the latter year or. September 1 1 (J. Rahts, 

 As/. Nach., No. 2700). It has again this year been observed 

 in a position closely agreeing with that computed from the data 

 obtained in 1885,50 that the new values for its elements are 

 probably very nearly correct. 



Variable Siak.s. — Harvard Cotltf^c Ohservalory Circular 

 No. 41 deals with the results of the photometric measurement of 

 the stars + 20" 4200 (U Vulpoculx) and + 28° 3460 (S.T. 

 Cygni), which were announced to be variable by Milllcr and 

 Kcmpf (Asl. Nach. , Bd 146, No. 37). The measures were made 

 by Prof. O. C. Wendell with the photometer with achromatic 



• NO. 1534, VOL. 59] 



prisms attached to the 15-inch equatorial. In his case the stars 

 were directly compared with stars of known magnitude in their 

 immediate vicinity, while at Potsdam each was compared with 

 the standard by means of an artificial star, and this fact probably 

 accounts for the greater accordance among the Harvard figures, 

 Apart from this the smoothed curves of both observers agrei 

 fairly closely, as is shown in the plotteil light curves given in 

 the article. 



The period of + 20^4200 is 7 98 days, during which th« 

 magnitude varies from 69 to 76. The star + 28' 3460 has a 

 period of 3 '8 days, its magnitude changing from 6'55 to 7 '36. 



In the remaining part of the Circular, dealing with the variabli 

 S Antli;i, the remarkable accuracy attainable with the apparatus 

 is well shown. Thisstar hasa periodof 7h. 46 Sm. (the shortest 

 known e.\cept in the case of variables in cluster*), and it was 

 doubted whether its period ought to be doubled as was the case 

 with U Pcgasi. The differences in magnitude of S .\ntlia 

 at minimum and its comparison star for widely differing epochs 

 only varied by 004 of a magnitude, so that the period 

 variation as taken is correct, and the star is not of the same type 

 as fl Lyrije or U Pegasi. 



Relation ok Eros to Maks. — In a short article in ihi 

 Astronomische A'achrichtcn, I5d. 14S, No. 3542, Herr J^ 

 Bauschingcr. of the Berlin (Jbservalory, points out the import-| 

 ance of the discovery of the minor planet Kros with reference td 

 the relationship of Mars to the other planets. Hitherto MarS 

 has been regarded as a major planet, and the asteroids as the 

 remnant of a former planet existing between it and Jupiter. 

 Since the recent observation of the new asteroid it is possible 

 to regard Mars itself as having been included in the original 

 planet which filled the gap, this view being supported by the 

 facts of Mars having so small a mass and the great eccentricity 

 of its orbit. If this turn out to be true, we shall in future 

 have to speak of the " Planetoid-ring between the Earth and 

 Jupiter " in discussing the asteroids. 



MEASURING EXTREME TEMPERATURES.^ 

 'X'HE measurement of extreme temperatures is a subject ol 



■^ great theoretical interest, especially in connection with the 

 determination of the laws of radiation and of chemical dis- 

 sociation and combination. The temperature in each case is 

 the factor of paramount importance, and without means of 

 measuring the temperature there is no possibility of formulating 

 any rational theories. The subject possesses, in addition, a 

 powerful fascination for the experimentalist, on account of the 

 difficulty of the observations involved, and of the extremely 

 conflicting nature of the results obtained by different observers 

 and different methods of research. 



Temperalure of the Sun. 



Attempts have frequently been made to estimate the tem- 

 peratures of the electric. arc and of the sun, which may be 

 taken as examples of the most extreme temjieratures known to 

 science, and afford an illustration of the difficulties to be en- 

 countered, and of the methods available for attacking these 

 problems. A brief consideration and comparison of the results 

 will also serve to explain the causes of the remarkable dis- 

 crepancies existing in the estimates of such temperatures by 

 diflerent observers and different methods. 



In the case of the sun it is at once obvious that no terrestrial 

 thermometer can possibly be directly applied. The only avail- 

 able method is (i) to measure the intensity of the solar radi- 

 ation, and (2) to endeavour to deduce the temjierature by 

 determining the law of radiation at high temperatures. The 

 measurement of the intensity of the solar radiation is in itself a 

 sufficiently intricate problem, containing many elements of 

 doubt and difliculty : but by far the greatest st)urce of uncertainty 

 lies in the solution of the second part of the investigation, the 

 determination of the law of radiation. The origin of the dis- 

 crepancies thus imported into the results may be summed up in 

 the word " Extrapolation. " 



The melhoil of investigation necessarily consists in taking a 

 series of observations at temperatures within the laboratory 

 range of thermometry, from which to calculate an empirical 



1 Discourse delivered at Ihc Royal Institution, on March lo, by Prof. H. 

 I,. Callcndar, K.R.b. 



