July 1. 1898.] 



KNOWLEDGE 



133 



A discussion by Lieuteuant-General Sfcrachey, of the 

 daily curves of temperature made at Greenwich Observa- 

 tory for twenty years, shows that the time of afternoon 

 mean temperature throui^'hout the year is a little before or 

 after 7 p.m. In the summer the time of absolute minimum 

 is between the hours of 3 a.m. and 6 a.m. Sunrise in 

 December is about an hour and a half before the time of 

 mean temperature ; while in June it is more than four 

 hours earlier. In the former month sunset is rather more 

 than three hours before the time of mean afternoon 

 temperature, and in the latter it is about half an hour 

 after that time. 



Hcttcrs. 



[The Editor does not hold himself responsible for the opinionii or 

 statements of correspondents.] 



♦ 



To the Editor of KNOWLEDtiK. 



Deau Sir, — To designate the point nearest to the sun 

 of a planet s orbit the word " perihelion " is used. 

 Similarly for the moon the word " perigee," for a star 

 the word " periastron," and for one of Jupiter's satellites 

 the hybrid word " perijove." I do not know what the 

 proper words are in the cases of the satellites of Mars, 

 Saturn, Uranus, and Neptune. Would it not be better, 

 and more convenient, to have one word which could be 

 used in every case ; say, some word derived from the 

 Greek meaning " nearest to the focus " '? I have forgotten 

 (if I ever knew) the Greek for focus, but I suppose there 

 is sTme word. In default of a word for focus, "pericentre " 

 (KiyTfjov) might be used; "centre," of course, being used 

 in the sense of centre of force. Can you suggest a word '? 



Yours truly, 



ntth June. J. R. Holt. 



To the Editor of Knowledge. 



De.\r Sir, — Would you kindly correct, in the nest 

 number, the following mistakes which have been made, 

 mostly in the printing of the tables : — 



Page 11.5, second column, line 31, instead of " Bradley- 

 Draper," read " B. D. (Bonner Durchmustering)." 



Page 117, Table 9 is incomplete ; add — 



Limits in J. Limits in a. Q. «. 



+ 20° to -I- 60" 22" ia'" — 3'' 29™ 1-43 oTw 



3-29— 8 15 0-9U 317 



81.5 — 13-2.5 0-64 42-7 



13- i5 — 17-59 0-71 55 8 



17-59 — 22 42 107 48 4 



+ 60 to -I- 90" 00 —240 "1-64 47-5 



Page 118, Table 11 is omitted. Read- 

 Result for the uieiui \alue of r = 0"082 — 

 230 Stars of Type I. gave q = + 0"062. 

 338 ., Type II. „ ? = + 074. 



Page 118, second column, line 7, instead of " mean mag. 

 3-6," read " 3-9." 



Page 118, Table 12, the letters r and y, to be inter- 

 changed. 



Page 118, second column footnote, add these words — 

 " The necessity of a correction of the constant of precession, 

 alluded to before, is evident from the values of t in this 

 table." 



]'>y making these corrections, you will oblige, 



Yours sincerely, 

 Groningen, 3rd June, 1893. .J. C. Kapteyn. 



continuous diminution in the proportion of Sirian stars 

 as we pass outwards from the plane of the Galaxy toward 

 the galactic poles, such as would result from a continuous 

 diminution in photographic brightness, found by Prof. 

 Kapteyn. 



Without identifying myself with Prof. Kapteyn's position, 

 I may state as a simple fact that such a diminution in the 

 proportion of Sirian stars in passing from the Galaxy to 

 its poles does not occur ; but that a diminution does take 

 place as we move from a zone included between 30^ S. 

 galactic latitude and the southern edge of the Galaxy in 

 the direction of the galactic poles. The following table 

 shows the position : — 



Class. NA. P.-70''. 70^-50". 5o''3f)*.3ti''loN. edge Galaxy. S.adeeof j-j'-so". so'-yo". /q^.S.G.P. 



of Galaxy. Galaxy to 50". 



AB ASS 65-1 31-9 613 69-8 74-6 .58-0 48-6 SO-0 



EPG 20-2 r2-7 18-7 IS 6 15-8 ll-_> 20-6 2i-6 lS-4 



HtK 21-8 80-8 2S-4 18 8 12' t !:!•+ 19-1 2t-0 :iO 



The above table I formed last summer when constructing 

 charts of the Draper catalogue stars, described in the 

 paper " On the Distribution of Stellar Types in Space " 

 (Aatronoiinj and Asfro-I'hysirs, .January, 1893), in which 

 the stars were inserted according to spectrum ; and the 

 set from which the above proportions are taken excludes 

 all stars the " observed brightness " of whose spectra 

 falls below 6-5 on a scale of 4-0 = bright, 8 -0 = faint, 

 adopted by Pickering, 6-5 being the limit at which the 

 spectrum of the star could be deciphered reliably. 



The region recognized as the Galaxy is that of Proctor's 

 Atlas. 



As Pickering's galactic divisions " M " (Aimah Harr. 

 Col. (His., Vol. xsvi.. Table xli.) include 2801 stars against 

 1208 present in the Galaxy of Proctor's Atlas, the fact 

 that his percentages are 



A r E LI H I K' 



•63 U 05 16 



against my 



A F E CJ H 1 Iv 



•662 -112 -046 -124 



shows that the extension on each side of the Galaxy brings 

 a diminution in the proportion of Siriaus for " observed 

 brightness" 6'5, as compared with the more limited area 

 of the Galaxy used by me. The theory of galactic 

 condensation derives no support from the extension of the 

 galactic area, as Mr. Monck supposed. 



The following table for stellar magnitude down to 5-99 

 {Harr. Photow.) brings me into strict line with Mr. 

 Monck's argument, who is working at that limit : — 



To the Editor of Knowledge. 



Sib, — Mr. Monck expresses a doubt in his letter on 

 Sirian and Solar Stars," in your June issue, as to the 



= 1208 stars. 



The first table, however, furnishes a more satisfactory 

 induction, considering that the number of stars employed 

 is almost four times as great, and we have Prof. 

 Pickering's assurance of the reliability of the spectrum- 

 reading at the limit of " observed brightness " used. 



It will be seen that the diminution is not " continuous," 

 nor could it well be expected to be so, for the condensation 

 of Sirian stars about the Galaxy does not conform with any 

 symmetry to the lie of the galactic plane, not to speak of 

 extra-galactic condensations of a thoroughly local, though 

 extensive nature. In fact, comparison of the contents of 

 zones upon galactic co-ordinates, where the contents are 

 so unevenly distributed along the zone, as they are for the 

 stars of the Draper catalogue, can only serve to level the 

 inequalities of distribution which it is the set purpose of 

 such comparisons to detect. 



There is a zone to which the main line of condensation 

 of Sirians conforms, and that zone is the great belt of 

 bright stars. This accounts for the fact that the axis of 



