Dec. 25, 1879] 



NATURE 



183 





tree-ferns have ever been found in English eocenes. In the 

 same way on the evidence of three seeds, which he 

 supposes to be grape stones, and some cactus spines, we 

 read that " the trees of the ancient forest were evidently 

 festooned with vines, beside which the prickly Rotang- 

 palm twined its snake-like form." Indeed, in addition to 

 the error he committed in calling them miocene, all 

 Heer's determinations of the Bovey Tracey plants require 

 revising. 



The Alum Bay leaf bed, familiar to geologists for 

 twenty or thirty years, appears at last to have almost 

 given out, for the leaf bearing pipe-clay is washed away 

 to such an extent that a fortnight's stay scarce yielded a 

 dozen of the commoner leaves. The unusual rainfall has 

 also nearly obliterated the Hempstead section, and the 

 face of the hill resembles a glacier of mud, which has 

 carried trees and bushes, in place of rocks, into the 

 sea. A lady, my brother, and myself had the misfortune 

 to select that route home, returning from Gurnet Bay when 

 darkness was coming on. The only passage over the deep 

 and perfectly soft mud streams lay through the dead 

 brushwood which fringed them. The tide was high on 

 one side, and up the escarpment on the other lay mud 

 and brushwood of the most impenetrable character, 

 while with a tide still rising and darkness increasing, it 

 appeared as hopeless to attempt to retrace our steps as to 

 press on. 



The following, from my note-book, has even less con- 

 nection with fossil leaves, but the experience may be of 

 use to geologists visiting the district. 



At Alum Bay a large area of weathered chalk, usually sup- 

 posed inaccessible, can be explored without much danger, 

 for it is almost everywhere possible to descend to the sea- 

 level between the Needles and the beacon on Freshwater 

 Down. The face of the cliffs is traversed by numerous 

 faintly marked tracks, which it is difficult to suppose could 

 ever have been of service except to smugglers, for the 

 shore line is rocky and not used by fishermen. Those 

 who appreciate the bolder coast scenery of our white 

 chalk will be repaid by climbs even of 500 or 600 feet, to the 

 perfect solitude of the water's edge. If accompanied by 

 ladies, a rope will be found a proper precaution and useful 

 in lessening the exertion to them. One of the easiest 

 ways is directly under the beacon, and there is a path 

 down into Scratchells Bay, just inside the railing of the 

 fort, whence at low tide the second of the Needles can 

 be reached. 



At Bournemouth we had a rather narrow escape. I 

 foresaw that during this year's digging unusual caution 

 would be necessary, owing to the heavy and saturated 

 state of the cliff. I was obliged, however, to go through 

 some fifteen feet of sand to reach a lower bed from which 

 I expected to get pinnae of Goniopteris Bunburyi. I 

 had dug out a piece of this bed from end to end ; a 

 distance of about twenty feet by three or four feet wide ; 

 and the cliff above this narrow excavation consisted of 

 some fifteen feet of vertical coarse sand, capped by 

 indurated ironstone, and a thick black clay bed, above 

 which the cliff sloped away at an angle. To expose a 

 little more of the leaf bed we ventured at one point to 

 slightly undermine the vertically of the cliff, before re- 

 placing the sand and clay we had dug out. During a 

 pause for lunch sand fell twice upon the leaf bed cleared 

 for work and was shovelled off. On a sudden loose 

 pieces seemed to be falling all along the face of our pit, 

 and with no more warning than an impulse to throw 

 ourselves out of danger, huge boulders of clay and iron- 

 stone tore by — which from their weight were afterwards 

 immovable to us — our excavation was completely filled in, 

 and our tools still lie buried under the debris. I was' 

 helplessly buried for a few minutes up to my shoulders 

 in sand, anticipating another slip, which fell soon after 

 i was extricated. 



J. STARKIE C7ARDNF.R 



RECENT EXPERIMENTS ON RADIATION 

 T7XPERIMEXTS on radiation have a twofold interest. 

 -*--' Accurate measurements of the increase of radiation 

 due to an increase of temperature have of course a great 

 theoretical value, but in addition to this, there is the 

 practical question of a possible measure of temperature 

 by means of the radiation of a body. It is this practical 

 question with special reference to the temperature of the 

 sun which seems chiefly to induce experimenters to study 

 the subject with improved methods. It has led at any 

 rate Mr. Rossetti to furnish a most valuable contribution 

 to the study of radiation.' 



Newton was the first to give a formula connecting the 

 quantity of heat radiated by a body with the temperature 

 of the body ; but his formula was not sufficiently accurate, 

 and has been replaced by another first given by Dulong 

 and Petit. But Dulong and Petit's formula also breaks 

 down when the difference of temperature between the 

 radiating body and the inclosure is large. 



Mr. Rossetti, trying to improve on Dulong and Petit's 

 formula, deduces from his experiments the following for 

 the radiation of lampblack : — 



y = a T~-(T - 6) - b {T - 6), 

 where y is proportional to the thermal effect of the radia- 

 tion, a and b are constants, and 7"and 6 are the temperatures 

 of the body and the inclosure, as measured on the abso- 

 lute scale. This formula seems certainly to be as far 

 superior to Dulong and Petit's as this latter was to 

 Newton' s. The last term generally is but small compared 

 to the first, and Mr. Rossetti believes it to be due to the 

 effect of the surrounding air, although we do not quite see 

 how this can be. The following experiments prove how 

 accurately the formula may be made to represent the facts. 

 The constants a and b were obtained by measuring the 

 radiation of a Leslie's cube filled with water or mercury, 

 and gradually heated up to 300'. A piece of copper foil 

 covered with lampblack was then heated in a flame of 

 alcohol. The temperature of the flame lies between 390° 

 and 400' ; and two numbers obtained by means of the 

 above formula were found to lie between these limits. 

 The radiation of a red hot copper sphere was then deter- 

 mined, and its temperature independently measured by 

 means of a calorimeter. The temperatures obtained by 

 the two methods were ~62°-i and /63 e -6 respectively. 



In order to find the temperature of the copper sphere 

 account was of course taken of the emissive power of 

 copper as compared with lampblack. For this purpose, 

 Mr. Rossetti has invented an ingenious method to deter- 

 mine this emissive power of various metals at the tempera- 

 ture of the Bunsen flame. That a formula obtained by 

 means of experiments made between 0° and 3oo C. should 

 give such accurate results for a temperature of 760 is 

 already a good proof for the usefulness of the formula, 

 but Mr. Rossetti has pushed his verification even further. 

 A cylinder of oxychloride of magnesium was heated in a 

 flame of coal-gas and oxygen. The temperature was found 

 to be about 900°, and in a flame it was found to be 2,167° 

 and c,397 : in two experiments. Platinum melted easily 

 in the name, and hence the temperature could not have 

 been far wrong. 



Before Mr. Rossetti can apply his formula to determine 

 the sun's temperature, he has to determine the absorptive 

 effect of our atmosphere ; but we shall not enter here into 

 this part of the question. The sun' s effective temperature 

 is the temperature he would have, if he had the emissive 

 power of lampblack. Mr. Rossetti finds this effective 

 temperature by his formula to be a little below 10,000" C. 

 Taking account of the fact that the sun himself is sur- 

 rounded by an absorbing atmosphere, and accepting some 

 data given by Secchi for the amount of this absorption, 

 the temperature of the photosphere is found to be above 

 20,000° C. 



" RerJe Arc. dei ! incei (3) II. t Jan. iS 7 3. 



