NATURE 



[November 5, 1891 



li might be supposed that tlie estimations would agree better 

 when the angles of position are the same for both arcs compared 

 together, than when they are different. But this supposition is 

 not borne out by my observations ; for after correcting them by 

 the above formula, the average deviation from the truth in the 

 case of the careful comparisons is 4*4 per cent, when the angle 

 of position of both arcs on the retina is the same, or within 10° 

 of the same; and 4"i per cent, when it difiers more than 10° ; 

 while in the case of the instantaneous comparisons these numbers 

 are 7*9 and 6 '3 respectively. 



VVhen the lower arc is horizontal, or nearly so, it is (on the 

 average) estimated as being shorter than when in a vertical posi- 

 tion, but the difference is so slight that it is doubtful whether it 

 would not disappear with a larger number of observations. The 

 best correction formula 1 have obtained for this is to apply the 

 factor 



(1-04 - -048 cos d) 



to the result 'already obtained : d being the deviation of the 

 lower arc from the horizontal. But the application of this factor 

 only reduces the sum of the squares of the differences between 

 calculation and observation in the case of the careful compaiisons 

 from 1 163 to nil. 



The angle of position of the upper arc seems to make no 

 difference in the results. T. W, Backhouse. 



West Hendon House, Sunderland, October 24. 



Proper Motions of the Stars. 



Miss Clerke, in her very interesting article (Nature, vol. 

 xliv. p. 572) on the moiion of th:: sun in space, seems to 

 think that we have only the two alternatives of supposing 

 that the brightness of a star is independent of its distance, 

 or that the motions of the stars increase with their distance. 

 I suspect that, when the proper motions of all stars down 

 to the 9th magnitude have been tabulated, the necessity of 

 adopting either alternative will disappear. My object in writing 

 this letter, however, is to call the attention of spectroscopists 

 to the question thus raised. The spectroscope, when used in 

 connection with a powerful telescope, ought to be able to show 

 whether the fainter stars as a rule move more rapidly in the line 

 of sight than the brighter ones ; for if the average motion in the 

 line of sight is the same in both cases, astronomers will be slow 

 to accept an explanation of phenomena which supposes a different 

 average velocity on the whole. But even instruments incapable 

 of deciding this question may throw light on the subject. It 

 now appears certain that if a Sirian and a solar star of the same 

 mass were placed at the same distance from us, the Sirian star 

 would appear more than one magnitude brighter. Hence, before 

 we can use magnitudes as in any sense a test of distance, we must 

 ascertain the relative proportion of Sirian and solar stars in the 

 groups which we are comparing. It would also be very desirable 

 that the magnitudes of the stars employed by Profs. Eastman, 

 Boss, and Stumpe, should be photometrically determined. The 

 photometer has at all events the advantage over the eye that 

 its results are in all cases (allowing for errors of observation) 

 comparable. W. H. S. Mo\CK. 



Dublin, October 17. 



California Foxcj. 

 In Nature of September 10 (p. 452), there is a paragraph in 

 praise of the intelligence of the (English) fox, with examples in 

 proof. Permit me to say that his California cousin is next door 

 to a fool. My young son has amused himself for the past three 

 summers in trapping (in large box-traps) the small California 

 foxes which infest these mountains, and which live on a mixed 

 diet of Manzanita berries and astronomer's chickens. I pass 

 over the fact that each trap has painted over its door " Danger 

 to all who enter here ! ", and I proceed to show that our California 

 foxes are barely one remove from idiots. When they are caught, 

 my boy is in the habit of fastening a small leather collar about 

 their necks, and of chaining them with light chains to stakes near 

 the Observatory buildings. Many of them have escaped by part- 

 ing the chains (by dint of strength, not of intelligence), and have 

 been again caught within two or three days in the same traps ! 

 One of them was caught three times in quick succession ! I pre- 

 sume an English fox, once caught, would emigrate to North 

 Britain, or at least to the next county. My own ideas of the 

 intelligence of the fox, until I came here, were derived from 

 Goldsmith's "Animated Nature," and, generally, from English 

 writings. 



NO. II 49, VOL. 45] 



I have now become satisfied that the California fox is arriire. 

 Either the struggle for existence is not sharp here, or he has 

 made up his mind that existence is not worth struggling for. 



Lick Observatory, October 8. Edward S. Holden. 



A Plague of Small Frogs. 



My wine-cellar has been visited during the recent rains with a 

 curious plague of small frogs {Rana te?nporaria) all the same 

 size, about one inch long. There would be nothing surprising 

 in this visitation were it not for the apparent absence of any 

 means of communication from outside, the level of which is 

 six feet above that of the cellar ; there is no drain near that part 

 of the house. There is a step up before you go down into the 

 wine-cellar from the adjacent cellar, against which the door 

 closes, leaving no crack any animal so large could squeeze 

 through. The cellar has solid stone walls and a bricked floor. 

 During the recent floods the water stood some three or four 

 inches deep there, apparently oozing through a tiny hole level 

 with the floor on the outside wall, into which the point of a 

 pencil could only penetrate for an inch. Even had it been possible 

 for these little creatures to come in that way they must have 

 burrowed down six feet from the outside level. Only one or 

 two were found in the cellar adjacent, which is lighted by a 

 grating into the garden, whereas in the wine-cellar two or three 

 dozen were caught, many of them drowned by the flood. 



Is it not unusual for bats to fly in the day-time ? Here one 

 has been doing so for two afternoons, coming out about 2.30, and 

 flying backwards and forwards after insects in most brilliant 

 sunshine. The gardener tells me he has never observed them do 

 so before ; and having sometimes caught them in the day-time, 

 always noticed that when thrown into the air they would drop 

 at once, and run instead of flying. 



R. Haig Thomas. 



BOTANY OF THE EMIN RELIEF 



EXPEDITION. 



''pHE botanical exploration of Tropical Africa leaves 



-^ so much to desire that it was somewhat disappoint- 

 ing to find that Mr. Stanley brought nothing back which 

 would give any idea of the nature of the dense forests 

 which he traversed. The conditions under which such 

 an expedition is necessarily executed make natural- 

 history-coliecting extremely difficult. Travellers, how- 

 ever, often suppose that because they cannot make 

 extensive collections they can do nothing to add to our 

 knowledge. Yet to fill a small portfolio with well-selected 

 and significant specimens is not a very difficult matter. 

 And these may often furnish the basis of useful and im- 

 portant conclusions as to the general nature of the flora. 

 Sir Joseph Hooker was able to give the first account of 

 the vegetation of Kilimanjaro from a small parcel of 

 plants collected by a missionary, the Rev. Mr. New, who 

 was supplied for the purpose by Sir John Kirk, with "a 

 bundle of old Guardians." An ofTicer of the Ashanti 

 Expedition brought from Comassi the fruit of what proved 

 to be a new species of Duboscia. And quite lately Lord 

 Lamington sent to Kew a small parcel of plants collected 

 by himself in an expedition through the Shan States, 

 which contained good specimens of an interesting plant 

 only known previously from imperfect material collected 

 by Griffith. It has now been worked out and figured in 

 the Kew " Icones Plantarum." 



Nor is it so difficult as it might be supposed to do even 

 more than this. And I am not sure that a little careful 

 and intelligent plant-collecting would not be a healthy 

 and useful distraction to the tedium and strain of an 

 arduous journey. Nothing could probably exceed the 

 difficulties under which Joseph Thomson travelled in 



Masailand ; yet he managed, notwithstanding, to get 

 together a tolerably extensive and most valuable botanical 

 collection. Upon this Sir Joseph Hooker was able to 

 base the first attempt at a rational theory of the geo- 

 graphical relations of the high-level flora of Eastern 



