May z, 1877] 



NATURE 



13 



friction will greatly interfere with its movement ; it must 

 not expose much surface, or it will be too heavy ; and it 

 must be a very bad conductor of heat, so as to retain the 

 excess of pressure on one side. Asrain, the part corre- 

 sponding to the cooler of the engine (the side of the glass 

 bulb) admits of but little modification. It must almost 

 necessarily be of glass, by no means the best material for 

 the purpose ; it is obliged to be of one particular shape ; 

 and it cannot be brought very near the driving surface. 



h perfect instrument would be one in which the heater 

 was stationary ; it might then be of the most suitable 

 material, of sufficient area of surface, and of the most 

 eftkicnt shape, irrespective of weight. The cooler should 

 be the part which moves ; it should be as close as pos- 

 sible to the heater, and of the best size, shape, and weight, 

 for utilising the force impinging on it. By having the 

 driving surface of large size and making it of a good con- 

 ductor of heat, such as silver, gold, or copper, a very faint 

 amount of incident radiation suffices to produce motion. 

 The black surface acts as if a molecular ' wind wei-e 

 blowing fi'om it, principally in a direction normal to the 

 surface. This wind blows away whatever easily movable 

 body happens to be in front of it, irrespective of colour, 

 shape, or material ; and in its capability of deflection 

 from one surface to another, its arrest by solid bodies, and 

 its tangential action, it behaves in most respects hke an 

 actual wind. 



Whilst the radiometer admits of but few modifications, 

 such an instrument as the one here sketched out is 

 capable of an almost endless variety of forms ; and as it 

 is essentially different in its construction and mode of 

 action to the radiometer, I propose to identify it by a 

 distinctive name, and call it the Otheoscope {w6ia, I 

 propel). 



The glass bulb is an essential portion of the machinery 

 of the radiometer, without which the fly would not move ; 

 but in the otheoscope the glass vessel simply acts as a 

 preserver of the requisite amount of rarefaction. Carry a 

 radiometer to a point in space where the atmospheric 

 pressure is equal to, say, one millimetre of mercury, and 

 remove the glass bulb ; the tly will not move, however 

 strong the incident radiation. But place the otheoscope 

 in the same conditions, and it will move as w'eil without 

 the case as with it. In the preliminary note already 

 referred to,- I described a piece of apparatus by which 

 I was able to measure the thickness of the layer of 

 molecular pressure generated when radiation impinged 

 on a blackened surface at any degree of exhaustion. At 

 the ordinary density of the atmosphere the existence of 

 this molecular disturbance was detected several milli- 

 metres oft", and its intensity increased largely as the 

 generating surface and movable plate were brought closer 

 together. It would be possible, therefore, to construct an 

 olheoscope in which no rarefaction or containing vessel 

 was necessary, but in which motion would take place in 

 air at the normal density.' Such a heat-engine would 

 probably work very well in sunlight. 



Aided by the mechanical dexterity of my assistant, Mr. 

 C. H. Gimingham, I have constructed several varieties of 

 otheoscope. These I propose to exhibit at the soiree of 

 the Royal Society on Wednesday next, as illustrations 

 of the very beautiful manner in which, at this stage of my 

 investigations, theory and experiment proceed hand in 

 hand, alternately assisting each other, and enlarging our 

 knowledge of tho^e laws of molecular movement which 

 coii.^tiiute a key to the relations of force and matter. 



e following is a list of the otheoscopes I have 



. cuiar, not molar. There is no wind in the sense of an actual trans- 

 it i.-cuf air from one place to another. This molecular movement may 

 be C'.iiipared to the movement of the gases when water is decomposed by an 

 electric current. In the water connecting the two poles there is no apparent 

 mov-ment, altliough eight times as much matter is passing one way as the 



'. Royal Soc. November 16, 187G, p. 310. 



> _■ writing this I have constructed such an instrument. The move- 

 ':'js place ill the w.ay I had anticipated. — W. C, April 26, 1S77. 



already made, together with some nevv experimental 

 radiometers, which will be exhibited for the first time 

 on Wednesday : — 



1. Otheoscope. — A four-armed fly carrying four vanes of 

 thin clear mica is mounted like a radiometer in an ex- 

 hausted glass bulb. At one side of the bulb a plate of 

 mica blacked on one side is fastened in a vertical plane in 

 such a position that each clear vane in rotating shall pass 

 the plate leaving a space between of about a millimetre. 

 If a candle is brought near, and by means of a shade the 

 light is allowed to fall only on the clear vanes, no motion 

 is produced ; but if the light shines on the black plate 

 the fly instantly rotates as if a wind were issuing from 

 this surface, and keeps on moving as long as the light is 

 near. 



2. Otheoscope. — A four-armed fly carries roasted mica 

 vanes and is mounted in an exhausted glass bulb like a 

 radiometer. Fixed to the side of the bulb are three plates 

 of clear mica equidistant from each other in a vertical 

 plane, but oblique to the axis. A candle brought near the 

 fixed plates generates molecular pressure, which, falling 

 obliiuelv on the fly, causes it to rotate. 



3. Otheoscope. — A large horizontal disc, revolving by 

 the molecular disturbance on the surface of inclined 

 metallic v.ines, which are blacked on both sides in order 

 to absorb the maximum amount of radiation. 



4. Otheoscope. — Inclined aluminium vanes driven by 

 the molecular disturbance from the fixed black mica disc 

 below, blowing (so to speak) through them. 



5. Otheoscope. — A large horizontal coloured disc of 

 roasted mica, driven by inclined aluminium vanes placed 

 underneath it. 



6. Otheoscope. — A bright aluminium disc cut in seg- 

 ments, and each segment turned at an angle, driven by 

 a similar one below of lampblacked silver. 



7. Radiometer. — A vertical radiometer, made with eight 

 discs of mica blacked on one side, and the whole sus- 

 pended on a horizontal axis which works in two glass 

 cups. The motion of the radiometer is assisted on each 

 side by driving vanes of aluminium blacked on one side. 



8. Radiometer. — A vertical turbine radiometer, the oval 

 vanes of roasted mica blacked on one side. 



9. Radiometer. — A spiral radiometer of roasted mica 

 blacked on the upper side. 



10. Radiometer of large size, showing great sensitive- 

 ness. 



11. Radiometer. — A two-disc radiometer, the fly carry- 

 ing roasted mica discs blicked on one side ; in front of 

 each blacked surface is fixed a large disc of thin clear 

 mica. The molecular disturbance set up on the black 

 surface, and streaming from it, is reflected in the oppo- 

 site direction by the clear plate of mica, causing the fly 

 to move abnormally, i.e., the black surface towards the 

 light. 



12. Radiometer. — A two-disc radiometer, the fly carry- 

 ing roasted mica discs blacked on one side, similar to 

 No. II, but with a large clear disc on each side. The 

 molecular disturbance, prevented from being reflected 

 backwards by the second clear disc, M thus caused to 

 expend itself in a vertical plane, the result being a total 

 loss of sensitiveness. 



13. Radiometer. — A two-disc, cup-shaped, aluminium 

 radiometer, facing opposite ways ; both sides bright. 

 Exposed to a standard candle 3'3 inches off, the fly rotates 

 continuously at the rate of one revolution in 3'37 seconds. 

 A screen placed in front of the concave side so as to let 

 the light shine only on the convex surface repels the latter, 

 causing continuous rotation at the rate of one revolution 

 in T$ seconds. When the convex side is screened oft", so 

 as to let the light shine only on the concave, continuous 

 rotation is produced at the rate of one revolution in 6'95 

 seconds, the concave side being apparently attracted. 

 These experiments show that the repulsive action of ra- 

 diation on the convex side is about equal to the attractive 



