3H 



NATURE 



[December 26, 19 18 



types of measuring machines. The sections deal- 

 ing with micrometers, and verniers are arranged 

 in a specially helpful manner, and contain many 

 diagrams showing the instruments set to various 

 readings, including those rases which present 

 difficulties to beginners. There is also a good 

 description of the Johansson system of standard 

 gauges and the methods of using these gauges, 

 and there is sufficient matter included on the 

 subjects of limits, tolerance, and limit-gauges. 



Omissions are inevitable in a small book of this 

 kind, and there are many special appliances de- 

 veloped during the war which do not find a place. 

 The success which the author has attained in this 

 and his companion hook for munition workers 

 should encourage him to undertake a more com- 

 prehensive volume in which laboratory as well as 

 workshop methods might find a place. 



LETTERS TO THE EDITOR. 

 [The Editor does not hold himself responsible jor 

 opinions expressed by his correspondents. Neither 

 can he undertake to return, or to correspond with 

 the writers of. rejected manuscripts intended for 

 this or any other part of Nature. No notice is 

 taken of anonymous communications.] 



Fuel Economisers. 

 Dr. Aitken's letter in Nature of December 12 rail-. 

 to iiu mind some unpublished experiments which 1 

 made at the National Physical Laboratory in con- 

 nection with some work mi radiation from surfaces, 

 which entirely bear out his statement that tin- coloui 

 of a hot surface at relatively low temperature has 

 very little influence on the amount of radiation leaving 

 it. This is a conclusion which, a priori, one would 

 not consider very probable, yet is actually found. 



In a series of experiments the side- of athin cubical 

 metal canister were painted in panels of varying 

 colour, the interior of the cube being filled with rapidly 

 stirred oil electrically heated. The amount of pure 

 radiation leaving each kind of surface at a series of 

 steady temperatures up to about 200° C. was com- 

 pared with that coming at the same temperature from 

 a "black body" constituted by a re-entrant tube with 

 appropriate diaphragms. The exterior of the tube was 

 washed by the hot oil. For temperature differences in 

 the region of ioo° C. it was found that: — 



(1) A bright surface of ordinary tin-plate only gave 

 off an amount of radiation equal to 5-10 per cent, of 

 that from a "black body." The quality of the optical 

 perfection of the surface was of little importance so 



it was bright. A metal surface treated with 

 galvanit of various kinds showed effects of the same 

 order as tin-plate. Burnished topper well cleaned with 

 metal polish gave a lower intrinsic radiation than tin. 



(2) A coat of almost an} paint, regardless of colour, 

 brings the true radiation up to from 80-90 per cent, of 

 thai of a "Hack body," and a quite thin laver of 

 papei varnish or of cel'uloid varnish, so thin and 

 transparent as to be almost imperceptible to tin eye, 

 applied over the bright metal, has almost the same 

 effect. 



(3) A layer <>l tissue-paper or wallpaper pasted over 

 the bright surface, or a coating of whitening or lime- 

 wash, shows the same kind of effect in' restoring 

 practically the full radiation so long as the coating is 

 thin. 



(4) If the surface of the cubi be metallised with 

 aluminium paint, the pun radiation is reduced to 

 from 45-55 P er cent - °f 'hat of a "black body." Much 

 depended, however, on the kind of vehicle used foi 



NO. 2^f>^. VOL. I02l 



the aluminium, and different samples of aluminium 

 paint, though giving results similar in appearand 

 thi eye, diffei considerably in the effects produced. 

 Bronzing and such-like processes produce intermedial 

 effects. 



In some later experiments, with which Mr. I 

 Griffiths was associated, a study was mad. ol th( 



total heat-leaving surfaces, with the view of obtaining 

 some data as to the relative (fleets of conduction", 

 convection, and radiation in ordinary still air. From 

 these experiments it would appear that in the case 

 of low-pressure steam radiators in the region of 

 ioo° C, almost exactly half the heat leaving the \. 1- 

 tical surfaces, if these are of an ordinary character 

 or painted in the usual manner, consists of pure 

 radiation, the remainder being the combined effect of 

 conduction and convection. Therefore, if, as is a 

 common practice, the radiators be metallised bv 

 painting with aluminium paint, the amount of heat 

 reaching the middle of a room warmed by such 

 radiators would be lowered to half, or double the 

 amount of heating surface would be required to pro- 

 duo the same radiation effect as if the surface were 

 black or of bare metallic iron. These results cer- 

 tainly have an important bearing upon the practical 

 problems of heat transfer. 



In the course of a recent perusal of Leslie's "On 

 Heat" I have been much struck with the fact that 

 many of these things were bv no means new to him 

 a hundred years ago. His eery suggestive and 

 interesting researches do not appear to be anything 

 like so well known as they ^certainly desi rve. 



J. A. Harker. 



Munitions Inventions Department, 



Princes Street, Westminster, S.W.i. 

 December 17. 



My attention has been directed to the note in N 

 of November 28 (p. 249) referring to Prof. C. V. 

 Boys's fuel economisers, and also the letter bv Dr. 

 John Aitken in the issue of December 12; and as I 

 devoted some consideration to this question about 

 twenty years ago, a description of the apparatus I 

 then devised for a similar purpose, and the results 

 obtained with it, may be of interest. 



In order to heat this house, which is a cold one, 

 and finding that the open fireplace in the hall con- 

 sumed much fuel with little heating effect, I fixed an 

 open fire-stove in front of the existing fireplace. I 

 then fitted a wrought-iron closed box, 3 ft. high, 2 ft. 

 broad, and 9 in. deep, at the back id the stove in tin 

 recess in which the fireplace previously existed. 



'fhe chimney of the stove was connected to the box 

 near the top at one side, and an outlet connected to 

 the box on the other side, which was led up th< 

 chimney. As this outlet-pipe is much smaller than 

 the existing chimney, an iron plate was fixed across 

 the intervening space so as to block it against thi 

 entry of air except through the stove. 



This box was divided vertically into two equal com- 

 partments by a plate extending from the top to within 

 6 in. from the bottom, so as to ensure that the hoi 

 gases flowed past the internal surfaces of the box and 

 imparted their heat to them. At one side id the box 

 near the bottom was fixed a door in case it should bl 

 necessan to remove any sool that may have accumu- 

 lated in it. 



'fhe stove is lighted in the usual manner with a 

 little coal, and a coke-fire is maintained thereafter. 

 The iron box soon becomi hi ated bv the gases issuing 

 from the stove, which without the addition of thi 

 would have passed direct into the chimney, and the 

 air of the room circulating around its external sur- 

 f.e 1 - becomes heated, and that of the hall warmed. 



