392 



NA TURE 



[March i6, 1876 



retaining the standard candle 48 inches off, on the left 

 of the bar, the index is brought to zero by placing on the 

 right 



2 candles 67 in. off. 



I candle behind solution of sulphate of copper 6 „ 



„ „ alum plate 14 „ 



A small gas burner 113 „ 



These experiments show how conveniently and accu- 

 rately this instrument can be used as a photometer. By 

 balancing a standard candle on one side against any 

 source of light on the other, the value of the latter in 

 terms of a candle is readily shown ; thus in the last 

 experiment the standard candle 48 inches off is balanced 

 by a gas-flame 113 inches off. The lights are therefore 

 in the proportion of 48^ to 113-, or as i to 5^. The gas- 

 burner is therefore equal to 5^ candles. 



By interposing screens of water or plates of alum, and 

 so cutting off the dark heat, the actual luminosity is 

 measured. In addition to this, by interposing coloured 

 glasses or solutions, any desired colours can be measured 

 either against the total radiation from a candle, its lumi- 

 nous rays, or any desired colour. One coloured ray can 

 be balanced against another coloured ray, by having 

 differently coloured screens on either side. 



The variations in the luminosity of a " standard " 

 candle will cease to be of importance. Any candle may 

 be taken ; and if it be placed at such a distance from the 

 apparatus that it will give a upiform deflection, say of 

 100 divisions, the standard can be reproduced at any sub- 

 sequent time ; and the burning of the candle may be 

 tested during the photometric experiments by taking the 

 deflection it causes from time to time, and altering its 

 distance, if needed, to keep the deflection at 100 

 divisions. 



If the pith bar in this instrument be blacked on alter- 

 nate halves, an impetus given by a ray of light always 

 acts in the same direction of movement. A candle causes 

 it to spin round very rapidly until the suspending fibre is 

 twisted up, and the rotation is stopped by the accumu- 

 lated torsion. 



By arranging the apparatus so that the black and white 

 surfaces are suspended on a pivot instead of by a silk 

 fibre, the interfering action of torsion is removed, and the 

 instrument will rotate continuously under the influence of 

 radiation. To this instrument the author has given the 

 name of the " Radiometer." It consists of four arms of 

 very fine glass, supported in the centre by a needle-point, 

 and having at the extremities thin discs of pith lamp- 

 blacked on one side, the black surfaces all facing the same 

 way. The needle stands in a glass cup, and the arms 

 and discs are dehcately balanced so as to revolve with 

 the slighest impetus. 



In the " Proceedings of the Royal Society " last year, 

 the author gave a brief account of some of the earlier 

 experiments with these instruments. In the present paper 

 he enters very fully into the various phenomena presented 

 by them, and gives Tables showing the number of revo- 

 lutions made by the radiometer when exposed to a con- 

 stant source of light removed different distances from the 

 instrument. The law is that the rapidity of revolution is 

 inversely as the square of the difference between the 

 light and the instrument. 



"When exposed to different numbers of candles at the 

 same distance off, the number of revolutions in a given 

 time are in proportion to the number of candles, two 

 candles giving twice the rapidity of one candle, and 

 three, three times, &c. 



The position of the light in the horizontal plane of the 

 instrument is of no consequence, provided the distance 

 is not altered ; thus two candles, one foot off, give the 

 same number of revolutions per second, whether they 

 are side by side or opposite to each other. From this it 

 follows that if the radiometer is brought into a uniformly 



lighted space it will continue to revolve. This is proved 

 to be the case by experiment. 



The speed with which a sensitive radiometer will re- 

 volve in full sunshine is almost incredible. Nothing is 

 visible but an undefined nebulous ring, which becomes at 

 times almost invisible. The number of revolutions per 

 second cannot be counted, but it must be several hun- 

 dreds, for one candle will make it spin round forty times 

 a second. 



The action of dark heat {i.e., from boiling water) is to 

 repel each surface equally, and the movement of the 

 radiometer is therefore arrested if a flask of boiling water 

 is brought near it. The same effect is produced by ice. 



From some observations made by the author, it appears 

 probable that heat of a still lower refrangibility repels the 

 white more than it does the black surface. Many in- 

 stances are given of the radiometer revolving the reverse 

 way. Thus, breathing gently on the instrument will gene- 

 rally cause this effect to be produced. 



An experiment is described with a radiometer, the 

 moving parts of which are of aluminium, blacked on one 

 side. When exposed to the radiation from a candle, the 

 arms revolve the normal way. On removing the candle 

 they revolve the reverse way. Heated with a Bunsen 

 burner the arms revolve the normal way as they are 

 getting hot, but as soon as the source of heat is removed 

 and cooHng commences, rotation sets up in the reverse 

 way, and continues with great energy till the whole is 

 cold. It appears as if the reverse movement during the 

 cooling is equal in energy to the normal movement as it 

 is being heated. 



It is easy to get rotation in a radiometer without having 

 the surfaces of the discs differently coloured. An experi- 

 ment is described with one having the pith discs blacked 

 on both sides. On bringing a candle near it, and shading 

 the light from one side, rapid rotation is produced, which 

 is at once altered in direction by moving the shade to the 

 other side. 



The author describes many forms of radiometer, by 

 means of which the movements can be exhibited to a 

 large audience, or can be made to record themselves 

 telegraphically on a self-recording instrument. 



THE WATER SUPPLY OF THE METROPOLIS 



IN the concluding portion of his anniversary address 

 printed in the last number of Nature (p. 376), the 

 late president of the Geological Society severely criticises 

 the proposal of the Rivers Commissioners to supply 

 London with pure spring water. The Commissioners 

 advise that the drinking water of London should continue' 

 to be derived from its present sources, but that it should 

 be led away to its destination before it is mixed with the 

 sewage of Oxford, Reading, Windsor, and other towns, 

 and before it is fouled by the filthy discharges of paper 

 mills and by other disgusting refuse. 



Mr. John Evans thinks that it can hardly be believed 

 that such a proposal as this should have been brought 

 forward, involving, as he believes it would, if carried out, 

 the conversion of the " fertile meadows " of the Thames 

 Valley into " arid wastes," and the utter destruction of 

 " watercress beds, now of fabulous value ; " he adds that 

 " even the canals and navigable rivers will become liable 

 to sink and be lost in their beds." In predicting these 

 dire results, he doubts whether his "judgment is seriously 

 distorted," although he admits being deeply interested in 

 the water power of one of the threatened valleys, and 

 protests that no one can submit silently to an insidious (?; 

 attack upon his property. 



Having carefully studied for many years the hydrogra- 

 phical features of the Thames and other valleys, I have 

 no hesitation in saying that Mr. Evans's fears are, for the 

 most part, entirely unsupported by experience. Sterility 



