June 24, 1 875 J 



NATURE 



147 



OUR ASTRONOMICAL COLUMN 

 The Double Star 2 2120.— In the notes to the last 

 catalogue of measures of double stars of the late Rev. 

 W. R. Dawes, he remarks with reference to 2 2120, or, as 

 it has been frequently called, Herculis 210 (Bode) : "This 

 object discovered by Strove is undoubtedly a binary 

 system— the position varying in a retrograde sense, and 

 the distance diminishing." Notwithstanding this positive 

 opinion as to physical connection of the components by so 

 high an authority in this department of practical astro- 

 nomy, an examination of the path of the companion up to 

 the latest published measures of the Baron Dembowski 

 towards the end of 1870, or, it should be stated, through 

 a period of observation fifteen years longer than that upon 

 which the above opinion was expressed, does not support 

 the presumed binary character of the object, but on the 

 contrary, when the apparent fixity of the principal star is 

 considered, shows pretty decidedly that the variation of 

 angle and distance must be owing to proper motion of 

 the smaller one. In fact we may represent the measures, 

 from Struve's earliest in 1829, to Dembowski's in 1870, 

 by the following expressions : — 



rtTsin P = — i""68o6 - o"-i2044 (/ — 1850-0) 

 ^/cos P = + I 7259 - o -10250 (/ - 1850-0) 

 which formula; imply a secular proper motion of the 

 small star amounting to i4"-75 in the direction 226°-o; 

 they may doubtless be somewhat improved by complete 

 discussion of all the measures, and perhaps some one of 

 the astronomical readers of Nature may be able to say 

 how measures in the present year are represented. 



D'Agelet, Bessel, and Struve have meridionally ob- 

 served this star. 



The "Mirk-Monday" Eclipse, 1652, April 7--8.— 

 The following elements of this long-remembered eclipse 

 are founded upon the same system of calculation which 

 furnished so satisfactory an agreement between compu- 

 tation and observation in the eclipse of 17 15, lately detailed 

 in Nature : — 



Conjunction inR.A. April 7, at 23h. 4m. 33s. g.m.t. 



The sidereal time at mean noon April 8 was ih. 9m, 20s., 

 and the equation of time im. 38s. subtractive from mean 

 time. The middle of general eclipse April 7, at 22h. 

 2im. 30s. 



Hence the following points upon the central line in its 

 track over the north of Ireland and Scotland : — 



Long. 8 25 W., Lat. 50 21 N. Long. 4 8 W., Lat. 55 3 N. 



,, 7 50 51 o „ 2 47 ,. 56 28 



,,63 53 o „ I 20 „ 57 54 



„ 5 26 53 42 



At Carrickfergus, where Dr. Wyberd observed the 

 eclipse as described in the " Philosophical Transactions," 

 totality began at 22h. 8m. 34s. according to the above 

 elements, and continued only forty-four seconds. At 

 Edinburgh it commenced at 22h. 22m. 555., the duration 

 being 2m. 47s. with the sun at an altitude of 39° ; and at 

 Arbroath at 22h. 26m. 23s. with the same duration, these 

 being local mean times. Probably there may be other 

 accounts of this eclipse in existence than those commonly 

 quoted when " Mirk Monanday " is referred to. 



Diameters of the Planets.— We give the following 

 values of the apparent diameters of planets reduced to 

 the mean distance of the earth from the sun and of their 



true diameters in English miles, as being perhaps as 

 reliable as any that can be assigned from existing data. 

 They are founded in every case upon the measures which 

 from observational circumstances appear to deserve the 

 greatest weight, and in the reduction to true values the 

 solar parallax is taken 8"-875, and Clarke's diameter of 

 the earth's equator is adopted. It would of course be idle 

 to attempt to offer final numbers, where the difficulties 

 attending observations and the differences between the 

 results of the most experienced and favourably- circum- 

 stanced observers are so considerable. 



/; Miles. 



Mercury 6*35 ... 2,850 



Venus 16-95 ... 7,550 



Mars 9-305 ... 4,150, 



Jupiter, Equat... 197-47 ••. 88,200 Compression -1- 

 ,, Polar ... 184-76 ... 82,500 i ^ 15-54 



Saturn, Equat. ... 166-82 ... 74,1:00)^ . i 



„ Polar ... 148-50 ... 66,300 i ^^^"^P'^^^^'^^ ^gVio 



Uranus 68-57 ... 30,600 



Neptune 67-26 ... 30,050 



In fixing upon .the apparent diameters of the bright 

 planets it has been desired to adopt values which shall 

 represent the actual arc values that are presented by the 

 true diameters at the earth's mean distance. Many 

 observations would assign larger values, but undoubtedly 

 less trustworthy for computing real dimensions. As is 

 well known, preference in such case is to be given to 

 double-image over wire-micrometer measures, yet even if 

 we confine ourselves to the former mode of observation 

 we by no means secure great consistency of results. 



SOLAR HEAT AND SUN-SPOTS 

 n^HAT the rainfall of certain parts of the earth tends 

 -*■ to vary periodically with the sun-spots has been 

 shown with considerable probability by Messrs. Meldrum 

 and Lockyer, and Prof. Koppen * has detected a similar 

 tendency in the temperature of the atmosphere, most 

 distinctly shown (as might have been anticipated) at 

 stations in the tropical zone, f These discoveries indi- 

 cate that there is at least some ground for the truth of 

 Sir W. Herschel's surmise, that the heat emitted by the 

 sun undergoes a periodical increase and decrease, con- 

 currently with the varying disturbance of the solar 

 atmosphere, as evidenced by the number of spots and 

 prominences on his surface. ,But except Mr. Joseph 

 Baxendell, who has published two papers on the subject 

 in the fourth volume of the Transactions of the Literary 

 and Philosophical Society of Manchester (new series), I 

 am not aware that anyone has attempted to investigate 

 the more direct evidence afforded by observations of the 

 black-bulb thermometer. 



Mr. Baxendell's work was based on the observations 

 of the Radcliffe Observatory at Oxford, and a series made 

 at Eccles by Mr. Mackereth ; the tv^o series extending 

 over the years 1859-66. He did not attempt to institute 

 a direct comparison of the recorded radiation tempera- 

 tures with the number of the sun's spots, a proceeding 

 which would probably have failed to lead to any definite 

 result in English latitudes and in so cloudy a climate ; 

 but took as the first term of his comparison the ratio 

 existing between the excess of the maximum radiation 

 over the maximum air-temperature in the shade, and 

 that of the mean air-temperature over that of the wet 

 bulb, or the dew point deduced therefrom. His conclu- 

 sion was to the effect that the sun's heat undergoes a 

 distinct periodical variation, coinciding with and directly 

 as that of the spots. 



* Zeitschrift der Oesterr. Gesellschaft fur Meteorologie. Vol. viii. Nos. 16 

 and 17. 



t It is to be noticed as a remarkable fact that Prof. Koppen finds the epochs 

 of maximum and minimum temperature to correspond (not .-is might be 

 expected from the results of the present investigation with the maxima and 

 minima of sun-spots respectively), but approximately with the opposite 

 phases; iht tnaximum of temperature (in the Uopics) preceding the mini- 

 mum of the sun-spots by o'^of a year, and the minimum of the former the 

 maximum of the latter by o'l year. 



