Sept. 25, 1884] 



NA TURE 



523 



is the main object of the Committee. A large amount of in- 

 formation is already in hand, much of which has been supplied 

 by Mr. J. B. Redman, who for many years has devoted special 

 attention to this subject. Mr. R. B. Grantham has also made 

 important contributions respecting parts of the south-eastern 

 coasts. But this information necessarily consists largely of local 

 details, and it has been thought better to defer the publication 

 of this for another year. Meanwhile the information referring 

 to special districts will be made more complete, and general 

 deduction- may be more safely made. As far as possible the 

 information obtained will be recorded upon the six-inch maps of 

 the Ordnance Survey. These give with great accuracy the con- 

 dition of the coast, and the position of every groyne, at the 

 time when the survey was made. 



Appended is a copy of the questions circulated. The Com- 

 mittee will be glad of assistance, from those whose local know- 

 ledge enables them to answer the questions, respecting any part 

 of the coast-line of England and Wales. Copies of the forms 

 for answering the questions can be had on application to the 

 Secretaries. 



Appendix — Copy of Questions. — I. What part of the English 

 or Welsh coast do you know well ? 2. What is the nature of that 

 coast? (a) if cliffy, of what are the cliffs composed? {/>) what are 

 the heights of the cliff above H.W.M. ? Greatest, average, least. 

 3. What is the direction of the coast-line? 4. What is the 

 prevailing wind ? 5. What wind is the most important — 

 {a) in raising high waves ? (6) in piling up shingle ? (e) 

 in the travelling of shingle? 6. What is the set of the tidal 

 currents? 7. What is the range of tide? Vertical in feet, 

 width in yards between high and low water, at spring tide, and 

 at neap tide. 8. Does the area covered by the tide consist of 

 bare rock, shingle, sand, or mud? (a) its mean and greatest 

 breadth ; (/<) its distribution with respect to tide-mark ; (c) the 

 direction in which it travels ; (d) the greatest size of the pebbles ; 

 (e) whether the shingle forms one continuous slope, or whether 

 there is a "spring full "and "neap full," if the latter, state 

 their heights above the respective tide-marks. 10. Is the shingle 

 accumulating or diminishing, and at what rate? II. If dimi- 

 nishing, is this due partly or entirely to artificial abstraction (see 

 No. 13)? 12. If groynes are employed to arrest the travel of 

 the shingle, state — (a) their direction with respect to the shore- 

 line at that point ; (/>) their length ; (c) their distance apart ; 

 (</) their height — (1) when built, (2) to leeward above the 

 shingle, (3) to windward above the shingle ; (e) the material of 

 which they are built ; (/) the influence which they exert. 13. 

 If shingle, sand, or rock is being artificially removed, state — 

 (a) from what part of the foreshore (with respect to the tidal 

 range) the material is mainly taken ; (/;) for what purpose ; [e) 

 by whom — private individuals, local authorities, public com- 

 panies ; (d) whether half-tide reefs had, before such removal, 

 acted as natural breakwaters. 14. Is the coast being worn back 

 by the sea ? If so, state — (a) at what special points or districts ; 

 (/<) the nature and height of the cliffs at those places ; (e) at 

 what rate the erosion now takes place ; (d) what data there may 

 be for determining the rate from early maps or other documents ; 

 [e) is such loss confined to areas bare of shingle? 15. Is the 

 bareness of shingle at any of these places due to artificial 

 causes? (a) by abstraction of shingle; (/<) by the erection of 

 groynes, and the arresting of shingle elsewhere. 16. Apart 

 from the increase of land by increase of shingle, is any land 

 being gained from the sea? If so, state — (a) from what cause, 

 as embanking salt-marsh or tidal foreshore ; (b) the area so 

 regained, and from what date. 17. Are there "dunes" of 

 blown sand in your district ? If so, state — (n) the name by 

 which they are locally known ; (6) their mean and greatest 

 height ; (<-) their relation to river mouths and to areas of shingle ; 

 (d) if they are now increasing ; (e) if they blow over the land, 

 or are prevented from doing so by " bent grass " or other vege- 

 tation, or by water channels. 18. Mention any reports, papers, 

 maps, or newspaper articles that have appeared upon this ques- 

 tion bearing upon your district (copies will be thankfully received 

 by the Secretaries). 19. Remarks bearing on the subject that 

 may not seem covered by the foregoing questions. [N.li. — 

 Answers to the foregoing questions will in most cases be ren- 

 dered more precise and valuable by sketches illustrating the 

 points referred to.] 



SECTION A— Mathematical and Physical Science 

 On Loss of Heat by Radiation and Convection as affected by 

 the Dimensions of the Cooling Body ; and on Cooling in Vacuum, 



by J. T. Bottomley. — In the course of a series of experiments 

 on the heating of conductors by the electric current, which were 

 carried on during the past winter, I obtained a considerable 

 number of results which both gave me the means of calculating 

 the eniissivity for heat in absolute measure of various surfaces 

 under different circumstances, and also caused me to undertake 

 a number of special experiments on the subject. These experi- 

 ments are still in progress, and I am making preparation for a 

 more extended and complete series ; but a brief notice of some 

 of the results already arrived at may not be without interest to 

 the British Association. 



The experiments were made on wires of various sizes, some 

 of them covered and some of them bare, cooling in air at ordin- 

 ary temperatures, and at normal and also at very much reduced 

 pressures. 



The mode of experimenting was as follows ; — 

 A current passing through a wire generates heat, the amount 

 of which is given by Joule's well-known law — 



H= C*RIJ (1) 



where C is the current, Ii the electrical resistance, J Joule's 

 equivalent, and H the quantity of heat generated per unit of 

 time ; each being reckoned in C.G.S. units. Let / be the length 

 of the wire, d its diameter, and a t the specific resistance of the 

 material at temperature t° (at which temperature let us suppose 

 that the wire in the given external conditions is maintained by 

 the current). Then 



y, _ at I _ \<itl 

 \itd- ~ W- 

 Hence from (1) — 



C- nail 



H = 



J 



(2) 



Consider, now, that the wire suspended in the air is losing heat 

 by its surface, and let us suppose that it neither loses nor gains 

 heat by its ends. Let II' be the quantity lost by emission from 

 the surface per unit of time. Let e be the eniissivity, or quantity 

 of heat lost per unit time per unit area of the cooling surface 

 per unit difference of temperatures between the cooling surface 

 and the surroundings ; and t° being, as has been said above, 

 the temperature of the wire, let 6 be the temperature of the sur- 

 roundings. Then 



H' =TcdI. e.(l - d) (3) 



But when the wire has acquired a permanent temperature, with 

 the current flowing through it, there is as much heat being lost 

 at the sides as is being generated by the current. In this case 

 ff= II' ; and we obtain the expression for e — 



e= 4C 7' (4) 



My experiments consist in measuring the strength of the cur- 

 rent and the temperature of the wire, the latter being effected by 

 measuring the electric re istance of a known length of the wire 

 while the current is flowing through it, and hence inferring the 

 temperature. These being known, and likewise the tempera- 

 ture of the surroundings, we have all the data for finding e, the 

 eniissivity of the surface in absolute measure. The experiments 

 of Mr. L>. Macfarlane giving emis-ivities in absolute measure 

 are well known, and are of undoubted accuracy. They were 

 communicated to the Royal Society (Proc. Roy. Soc, 1872, 

 p. 93) ; and the results are quoted in Prof. Everett's " Units 

 and Physical Constants" (chap. ix. § 137). These experi- 

 ments were made with a copper globe about 4 cm. in diameter, 

 suspended in a cylindrical chamber, with top and bottom, about 

 60 cm. in diameter, and 60 cm. high. The results may be 

 briefly summed up as follows : — 



Macfarlane finds an eniissivity of about i/4O00th of the 

 thermal unit C.G.S. per square centimetre per second per degree 

 of difference of te nperatures between cooling body and sur- 

 roundings for a polished surface, with an excess of te.nperature 

 of a little more than 60° C. ; and, for a blackened surface, the 

 same eniissivity with an excess of 5 C. or under. 



Using round wires of small diameter (0-85 mm. and under), 

 and with the surfaces either brightly polished or in common dull 

 condition of a wire fresh from the maker, I have found a much 

 larger emissivity than 1/4000. I have obtained different values of 

 e for wires of different sizes, varying from 1/2000 down to 1/400, 

 which was obtained with a wire of 0^40 mm. diameter, and with 

 an excess of temperature of 24° C. It seems to be shown by all 

 the experiments I have made that, other things being the same, 

 the smaller the wire the greater the emissivity. 



