Sept. 28, 1876] 



NATURE 



475 



and in any further investigation the first step must be to 

 ascertain under what interpretation these can best be re- 

 conciled with subsequent measures, it being evident that 

 all cannot be accepted as registered. Sir John Herschel 

 considered the angles of 1792 and 1795, especially the 

 former, must be affected with considerable error. These 

 angles are respectively 3S5°74 and 354°'9, or by a mean 

 355°*3 for 179376, but if we suppose that they should 

 have been registered in the north-following quadrant, 

 instead of the north-preceding one, the mean would 

 become 4°7, an angle in much better accordance with 

 the progression shown by the observations of 1782, 1802, 

 1804, and those of Struve, Herschel, and South about 

 1821. It might be worth while to determine how far this 

 alteration would lead to a more admissible orbit. At the 

 same time we have to bear in mind Sir W. Herschel's 

 remarks with respect to his observation, 1792, April 20, 

 in Phil. Trans., 1804, p. 367. And equally are we to 

 take into consideration for our guidance the same ob- 

 server's estimations of distance in 1782 and 1804. 



Diameter of Vesta. — At the opposition of 1855, this 

 brightest of the minor-planet group, which during the last 

 spring, as in previous favourable oppositions, was dis- 

 cernible with the naked eye, was observed by Prof. Secchi 

 to present a diameter but little inferior to that of the first 

 satellite of Jupiter, " ma molto piu debole di luce, e di 

 colore ranciato carico," and he estimated it at o"*8 ; this 

 we find corresponds to a true diameter of 450 miles. The 

 least distance of the planet from the earth in 1855 was 



1'26. 



Pigott's Comet of 1783. — On the night of November 

 19, 1783, a comet was discovered by our countryman, 

 Pigott, at York, well known as having also detected the 

 remarkable variable stars, R Coronse Borealis, R Scuti, 

 and 77 Aquilce. Pigott notified his discovery to Mechain, 

 who observed the comet at Paris on the 26th of the same 

 month, and, in conjunction with Messier, determined its 

 positions until December 21. It was at no time visible 

 without the telescope. Elements were calculated by 

 Mechain and Saron, though without satisfactory results 

 on a parabolic hypothesis. Subsequently Burckhardt in- 

 vestigated the orbit without this assumption, and finally 

 arrived at elliptical elements, with a period of revolution 

 of 5 6 1 years. But the most precise determination of 

 the orbit from the Paris observations has been made by 

 Prof. Peters, of Clinton, U.S., who reduced the observa- 

 tions anew, and introducing Hansen's Tables for the 

 earth's positions, found elements which "represent the 

 whole series to satisfaction." These elements are pub- 

 lished in the "Astronomical Notices" issued by Prof. 

 Briinnow, while in direction of the Observatory at Ann 

 Arbor, Michigan (No. 19), but as this periodical is 

 comparatively little known in this country, having been 

 continued for a short time only, we transcribe the orbit 

 here : — 



Perihelion Passage, 1783, Nov. 19 "93685 M.T. at Paris. 



Longitude of perihelion ... 

 „ ascending node 



Inclination 



Angle of eccentricity 



Log. semi-axis major 



Period of revolution 



50 17 25-4) 



55 40 30"5 \ M.Eq. 1783-0 



45 6 53-8 \ 



33 32 8-4 



0-5133056 

 5 -888 years. 



In this orbit the perihelion distance is 1*4593, ^'^d the 

 aphelion distance 5-062. 



The comet has not been found since 1783. As re- 

 marked by Prof. Peters, a major-axis differing but little 

 from the above would have sufficed to bring the comet 

 into close proximity to the planet Jupiter, at one or other 

 of the subsequent aphelion passages, whence it is possible 

 great perturbations may have resulted, even of magnitude 

 sufficient to effect an entire change of orbit. Indeed with 

 the above elements we find the distance of the comet 



when in aphelion, from the orbit of Jupiter, is only 0*42. 

 Independently of this, there is another cause which might 

 have long operated to prevent the re-discovery of the comet : 

 in 1783 it appeared under nearly the most favourable cir- 

 cumstances possible for observation, yet as before stated it 

 was at no time visible to the naked eye, and while 

 approaching pretty near the earth, did not exceed 8' in 

 diameter, presenting throughout the appearance of the 

 great majority of telescopic comets. 



The orbit of Pigott's comet passes very near to that of 

 the planet Mars : m heliocentric longitude S5°-2, we find 

 the distance is only 0-032, and it is to be remarked that this 

 close approach takes place in one of the regions where 

 the orbit of the lost comet of De Vico also comes into 

 such near proximity to that of the planet ; still after M. 

 Leverrier's statement with reference to past perturbation 

 of De Vico's comet by Mars, we are not to suppose that 

 the bodies can be probably identical. 



With respect to the introduction of Pigott's comet 

 into our system, small variation in the major-axis assigned 

 by Prof. Peters would have caused a verj- close approach 

 of the comet to Jupiter at the aphelion passage imme- 

 diately preceding the comet's appearance, or early in 

 1781. 



THE SELF-FERTILISATION OF PLANTS 



MR. THOMAS MEEHAN, one of the most acute 

 and thoughtful of American botanists, has several 

 times during ihe present year brought before the Phila- 

 delphia Academy of Natural Sciences the subject of the 

 fertilisation of plants. He has observed that there are 

 plants with conspicuous and attractive flowers, which are 

 as much adapted to secure self- fertilisation as other flowers 

 are for cross-fertilisation. One of his examples is the 

 green-house annual, Brewallia data, belonging to the 

 order Scrophulariaceae, having an attractive blue flower. 

 Not only does it produce abundance of perfect seeds 

 without insect aid, but also the entrance of an insect 

 would ensure self-fertilisation. The style is nearly as long 

 as the corolla-tube, and the slightly longer stamens are 

 arranged closely around it. Two of the anthers are 

 inverted over the stigma, and their connective is densely 

 bearded, appearing like petaloid processes, completely 

 closing the tube of the corolla. No insect can thrust its 

 proboscis into the tube except through this mass ; and if 

 it has foreign pollen adherent to it, it will be cleaned off 

 by the beard. Furthermore, the very act of penetration 

 will thrust the anthers forward on to the pistil, and aid in 

 rupturing the pollen sacs, and securing self-fertilisation. 



Another phenomenon, the " sleep " of plants, or closing 

 of the flowers at nightfall, has been found by Mr. Meehan 

 to have reference to self-fertilisation in Claytonia virgi- 

 nica (order Portulacaceae) and some buttercups, which seed 

 abundantly, without being visited by insects. In Clay- 

 tonia, the stamens, on expanding, fall back on the petals 

 expanded during daylight. At night, when the flower 

 closes, the petals cany the anthers into close contact with 

 the stigmas, and actual fertilisation only occurs in this 

 way. In many cases, the stamens recurve so much as to 

 be considerably doubled up by the nocturnal motion of 

 the petals ; thus the anthers are not brought into contact 

 with th? stigmas, and the flowers are barren. 



In Raminailus buibosus, in the evening following the 

 first day's expansion of the flower, Mr. Meehan has found 

 the immature anthers and the young stigmas covered 

 with pollen-grains. This would naturally be supposed to 

 be the consequence of insect visits ; but no insect visits 

 had taken place in the cases examined. However, on 

 carefully studying the flower it was found that coinci- 

 dently with its- expansion, a single outer series of stamens 

 shed iheir pollen into the petals, from which it easily 

 fell to the immature anthers and the stigmas, when the 

 flower closed for the night Another equally remark- 



