6o 



NATURE 



[June i6, 1898 



OUR ASTRONOMICAL COLUMN. 



Encke's Comet. — Of the three comets which are due to re- 

 turn this year — namely Encke's, Winnecke's and Wolf's, 

 having periods of 3^, 5^, and nearly 7 years respectively — the 

 first seems to have just been found, according to a Kiel tele- 

 gram dated June 14. Prof. Hussey, telegraphing to Kiel, states 

 that Mr. Coddington, on June 11, gh. I3'im. Lick mean time, 

 found a comet, which he terms bright, in position R.A. 

 l6h. 24m. 45'9s. and Declination (south) 25° 14' 20", the daily 

 motions in these coordinates being 51' and 36' respectively. 

 The comet thus lies in the constellation of Scorpio, a little to 

 the north of the bright star a. A further telegram from Mr. 

 John Tebbutt, dated June 14, states that this observer found the 

 comet on June lain position R.A. i6h. 21m. and Declination 

 (south) 25° 52' at gh. 22 "Qm. Lick mean time. 



Much interest is attached to this comet, since its period is 

 one of the shortest known. It was first seen in 1818 by that 

 diligent observei' Pons, on November 26, perihelion being passed 

 in the following January. It was Encke, however, who under- 

 took to investigate its motions, proving that its period extended 

 over 3^ years, and he predicted its return in 1822. At every 

 succeeding return the comet has been observed, and it was even 

 discovered that prior to 18 18 it had been three times observed 

 by Mechain, Caroline Herschel, and Thulis in the years 1786, 

 1795 and 1805. At its last return, in 1895, it was just visible 

 to the naked eye at the time of its maximum brightness. 



New Determination of the Earth's Density.— Herr 



F. K. Ginzel, in the current number of Himmel und Erde 

 (June, Heft 9), describes a new determination of the mean 

 density of the earth by Dr. C. Braun, a former director of the 

 observatory at Kalosca in Hungary. The apparatus used for 

 this purpose was a torsion balance constructed by Dr. Braun 

 himself, and from the description we learn that, excepting the 

 suspension wires, glass globe, chronometer, chronograph, micro- 

 scopes, and a few small parts, everything was made by himself. 

 The method employed differed mainly from previous determina- 

 tions in that the torsion balance was enclosed in a glass globe 

 from which all air had been extracted. So complete was the 

 vacuum that after four years no change could be detected. We 

 leave our readers to gather from the above-mentioned source 

 more details regarding the apparatus itself. The observations 

 were begun in the year 1892, and two years later the computa- 

 tions were commenced. After all allowance had been made for 

 corrections the final result gave for the value of the mean density 

 of the earth 5 '52765, which nearly corresponds with the best de- 

 termination made by Prof. Boys. Herr Ginzel, in concluding his 

 article, tells us that, disregarding the very high scientific 

 importance that will be attached to this new determination, if 

 we consider that Dr. Braun is considerably advanced in years, 

 somewhat hard of hearing, and has not been blessed with good 

 health during the last few years, and that he has been thinking 

 over this problem for eleven years in addition to his usual official 

 duties, all will agree in saying that this work is a rare proof of 

 the scientific energy and ideal power of sacrifice for one man. 



The Large Refractors of the World. — The question 

 of the efficiency of refractors of large apertures has recently 

 been discussed in many articles, and the latest we owe to Prof. 



G. E. Hale, who deals in Science (May 13) with the frequently 

 asked question, " Do large telescopes pay?" Prof. Hale points 

 out the special kind of work to which refractors of large -aperture 

 should be employed, and shows that when used by a skilled 

 observer very important work can be accomplished which would 

 be impossible with a small aperture. An instrument, say, of 

 forty inches aperture is more advantageous than one of ten 

 inches, in that, first, it has the power of giving much brighter 

 images, thus rendering faint stars visible. It can, secondly, 

 give an image of a celestial body of measurable dimensions 

 four times as large as that given by a lens of one-fourth its 

 aperture and focal length ; and, thirdly, its capacity of rendering 

 visible, as separate objects, the components of very close double 

 stars or minute markings upon the surface of a planet or satellite. 

 Prof. Hale concludes that all the money and time and labour are 

 well spent on refractors of large aperture, and he suggests that 

 further sums might well be expended, particularly in the southern 

 hemisphere, in the establishment of still more powerful in- 

 struments. 



A propos of large refractors, a fairly complete list of existing 

 large refracting telescopes appears in the current number of the 

 Observatory (June), in which are given details concerning the 



aperture, focal length, location, maker, and date of erection of 

 the various instruments. America comes first as regards the 

 number of instruments and largest size of aperture, followed by 

 France, England and Germany in the order respectively of 

 the number of refractors exceeding 13 '4 inches. 



The Leeds Astronomical Society.— It is always with 

 pleasure that we refer to scientific societies for the promotion 

 and extension of astronomical knowledge, when we know that 

 they are doing useful work in this respect. 'Y\\& Journal i^o. 5) 

 of the Leeds Astronomical Society for the year 1897 is a good 

 example of the interest displayed by its members in fostering 

 astronomy, and during the past year many interesting papers 

 were read at their meetings. Among these may be mentioned 

 that on the nebular origin of our solar system, by Mr. Barbour, 

 who refers to and extends the significant relationships between 

 the distances and masses of the four superior planets suggested 

 by Mr. Sutcliffe of Bombay, and previously referred to in this 

 column (vol. Ivi. p. 424). Other papers read had for their 

 subjects the heat of the sun, the planet Venus, orientation of 

 Egyptian temples, density of the earth, &c. This number con- 

 tains also an excellent likeness of the retiring president, Mr. 

 Washington Teasdale. 



RECENT EXPERIMENTS ON CERTAIN OF 

 THE CHEMICAL ELEMENTS IN RELA- 

 TION TO HEAT} 

 'X'HE discovery that different substances have different capaci- 

 *- ties for heat is usually attributed to Irvine, but there can 

 be no doubt that Black, Crawford, and others contributed to 

 the establishment of the idea. The fact that equal weights of 

 different substances in cooling down through the same number 

 of degrees give out different amounts of heat, may be illustrated 

 by the well-known experiment in which a cake of wax is pene- 

 trated with different degrees of rapidity by balls of different 

 metals heated to the same temperature. But for the quantita- 

 tive estimation of the different amounts of heat thus taken up and 

 given out again, the physicist must resort to other forms of 

 experiment, each of which presents difficulties of its own. 

 Broadly speaking, three principal methods have been used in 

 the past for the estimation of "specific heats." The first is 

 based upon the observation of the exact change of temperature 

 produced in a known mass of water by mixing with it a known 

 weight of the substance previously at a definite temperature 

 above or below that of the water. The second consists in 

 determining the quantity of ice melted when the heated body 

 is brought into contact with it in such a way that no heat from 

 any other source can reach the ice. And the third method 

 consists in observing the rate at which the heated body falls 

 through a definite range of temperature when suspended in a 

 vacuous space. 



The process of intermixture with water was used by the 

 earlier experimenters in the last century, and some of the best 

 results extant have been obtained by this process, which, how- 

 ever, is not so easy as it appears when the highest degree of 

 accuracy is desired. 



Lavoisier and Laplace in 1780 devised the ice calorimeter 

 which bears their name, and in a most interesting memoir, 

 which is reprinted among Lavoisier's works, they show that they 

 were familiar with the idea which in modern times is known as 

 the principle of the conservation of energy. In this memoir 

 they give the results of experiments in which the specific heats 

 of iron, mercury, and a few other substances are estimated with a 

 very tolerable approach to accuracy. Although many of the 

 metals were known to them, it would not have been possible, 

 had they persisted in this work, to make the discovery which 

 was reserved for Dulong and Petit thirty-five years later, for the 

 atomic theory had not been conceived and no atomic weights 

 had been determined. 



Dulong and Petit {Ann. Chim., 1817, vii. p. 144) seem to 

 have used at first the method of mixtures, and to have found by 

 direct experiment that the specific heat of solids (metals and 

 glass) increases with the temperature. They also studied (after 

 Leslie) the laws of cooling of bodies ; and two years after the 

 publication of their first paper on the subject, they (Petit and 

 Dulong, sic) arrived at the remarkable general expression which 

 is associated with their names {Ann. Chim., 1819, x. 395). 



1 A discourse delivered at the Royal Institution, Friday evening, May 13, 

 by Prof. W. A. Tilden, D.Sc, F.R.S. 



NO. 1494, VOL. 58] 



