August, 1903.] 



KNOWLEDGE 



183 



presume lie is alluding to the comet of 1824. Unfortunately 

 but little (lata is to band as to these comets. Photo- 

 graphy was not known. Were they to appear now, 

 sufficient data could undoubtedly be collected either to 

 confirm or disprove my theory. The only comet from 

 which rays were apparently lying on the line of the " radius 

 vector " was that of 1882, faint indications of which are 

 discernible in the photographic views of this comet on 

 November 7th, 1882. These photos were taken at the 

 Cape, and a faint ray running into the head of the comet 

 i-i distinguishable for a short distance from the head and 

 decreasiiuj iu width as the rays leave the comet. In my 

 own mind there is only one explanation for this luminous 

 appearance, and it is that there must have been a comet 

 travelling probably in the same ellipse and as an atten- 

 dant Ijody whose bead was lost in the brightness of the 

 sun but whose tail ran into the big comet on that date and 

 was thus caught in these photos. This may or may not 

 have been the case with the comets the reviewer wishes to 

 quote. We have not, however, sufficient data to determine 

 this point. 



In conclusion, I must protest against the remarks that 

 the reviewer has noticed several errors in my work. He 

 only quotes two of the said mistakes, one obviously a 

 printer's error, in which the year 1774 is printed instead 

 of the year 1744. The other is no error, namely that 

 Donatis comet was first discovered by Tuttle. I omitted 

 to say " in America," and the meaning I had must be 

 perfectly apparent by the text following the sentence, and 

 also from my using the European discoverer's name. No 

 other printer's errors exist and no other omission of words 

 can be laid to my door in the whole of my work. 



Fred. G. Shaw. 



[Mr. Shaw's letter can hardly be said to strengthen his 

 case. He says that he can recall no comet of the year 

 1823, but any astronomical work will inform him that a 

 bright comet passed its perihelion on December 9th of that 

 year. It was visible during the months of December, 

 January and February, 1823-4. Hind referred to this 

 comet as " remarkable for having exhibited a tail directed 

 towards the sun in addition to another in the usual 

 position." The fourth comet of 1851 presented a similar 

 aspect, its tail "consisting of two branches, one of which 

 was turned towards the sun." There are other instances, 

 in spite of Mr. Shaw's assertion that the comet of 1882 

 supplied the only case of the kind. Tour correspondent 

 remarks that his statement " Tuttle discovered Donati's 

 comet on June 2nd, 1858," was no error, only an omission 

 of the words " in America." He evidently does not 

 believe in errors of omission as well as errors of com- 

 mission ! Mr. Shaw's revival of old theories regarding 

 comets' tails is never likely to exercise any weighty 

 influence in explaining the phenomena recorded. The 

 l>ehaviour and occasional positions of the tails of comets 

 would certainly suggest that they are not due to the 

 action of refractive atmospheres surrounding the nuclei. 

 A theory to commend itself must agreeably accord with 

 observed facts, and will neither present serious dis- 

 cordances with them nor require us to adopt very 

 improbable assumptions. — The Reviewee.] 



Astronomical. — The important pari in cosmieal phe- 

 nomena which is probably [)layed by the prcssim- due to 

 radiation is receiving considerable attention at the present 

 time. In 1873 Clerk Maxwell showed that if light be au 

 electro-magnetic phenomenon, the absorption or reflection 

 of a beam of light should produce a pressure, and he 



further computed the amount of this pressure. A year 

 or two ago Prof. Lebeden, of Moscow, convinced himself 

 of the existence of such a pressure, and found from his 

 experiments that, within the limits of error, its amount 

 was equal to that theoretically arrived at by Maxwell. A 

 more complete investigation has recently been made by 

 Professors Nichols and Hull, of Dartmouth College, 

 U.S. A , and Maxwell's conclusion is again confirmed. It 

 appears also that the radiation pressure depends only 

 upon the intensity of the radiation, and is independent of 

 wave-length, so that the long invisible waves are as 

 effective in producing pressvire as are the visible radia- 

 tions. In attempting to apply this result to cosmieal 

 phenomena, it is to be borne in mind that while the 

 gravitational attraction on particles varies with the centre 

 of the radius, the repulsion due to radiation varies with 

 the square of the radius, so that the ratio of repulsion to 

 attraction will increase as the particles diminish in size ; 

 but a limit is reached when the particles are so minute 

 that the radiation pressure is dispersed through diffraction 

 phenomena. For this reason gases and vapours are not 

 subjected to pressure from radiation, and the maximum. 

 ratio of radiation pressure to gi-avitation is about 20 to 1. 

 The forms of comets' tails may be thus satisfactorily 

 accounted for, the form depending on the size or density 

 of the particles repelled from the head. Professors Nichols 

 and Hull also describe an experiment for illustrating the 

 formation of a comet's tail, and deal very fully with the 

 whole subject in the June number of the Astrophysical 

 Journal. — A. F. 



Botanical. — The last number of the Transactions of 

 the Linnean Society consists of au admirable contribution 

 to our knowledge of insular floras in Mr. T. F. Cheese- 

 man's " Flora of Rarotonga." Rarotonga is the chief 

 island of the Cook group, situated in the Eastern Pacific, 

 between the Tonga and Society Islands. Though dis- 

 covered iu 1821, and occupied by Europeans ne<irlv the 

 whole time since, scarcely anything has hitherto been 

 published on its flora. The present paper includes an 

 enumeration of the plants collected by the author during 

 a three mouths' visit, made at the most suitable time of 

 the year for studying the vegetation of the island. It is, 

 therefore, practically a complete flora of the Phanerogams 

 and Ferns. Rarotonga is of volcanic origin, and is about 

 eight miles long and six broad. Near the sea there is a 

 level tract of land, varjdng from a quarter of a mile to a 

 mile in width, extending all round the island, while the 

 centre is a mass of rugged mountains, ranging from 1575 

 to 2250 feet high. Forests aboimd, but the herbaceous 

 vegetation, except numerous ferns, is scunty. With the 

 inclusion of nearly one hundred species, which have with 

 more or less certainty been introduced, the flora as at 

 present known consists of three himdred and thirty-four 

 vascular plants, of which no less than sixty-seven are 

 ferns. Eighteen species belonging to sixteen natural 

 orders are probably endemic. Pei'haps the most remark- 

 able ])lant amongst these is Fitchia speciosa, a ne>v 

 species of a curious genus of Composita;, hitherto known 

 only from the Society Islands, where three species are 

 found. F. upeciosa is common on all the mountains of 

 Rarotonga. It is a small handsome tree, with fine glossy 

 leaves and large orange-red flower-heads. The flora of 

 Rarotonga resembles most closely that of the Society 

 Islands, having one huudred and sixty-seven species in 

 common, while one hundred and three of the same species 

 are also found iu the Tonga group. — S. A. S. 



Zoological. — In a paper recently read l>efore the 

 Linnean Societv, Mr. B. A. Benslev discusses the origin 



