70 Royal Society : — 



blackened surface at any degree of exhaustion. At the ordinary 

 density of the atmosphere the existence of this molecular disturbance 

 was detected several millimetres off, and its intensity increased 

 largely as the generating surface and movable plate were brought 

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

 otheoscope in which no rarefaction or containing vessel was neces- 

 sary, 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 will be exhibited 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 those laws of molecular movement which con- 

 stitute a key to the relations of force and matter. 



The following is a list of the otheoscopes I have already made, 

 together with some new 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 exhausted glass bulb. 

 At one side of the bulb a plate of mica blacked 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, equi- 

 distant from each other in a vertical plane, but oblique to the axis. 

 A candle brought near the fixed plates generates molecular pressure, 

 which, falling obliquely on the fly, causes it to rotate. 



3. Otheoscope. — A large horizontal disk revolving by the mo- 

 lecular disturbance on the surface of inclined metallic vanes, which 

 are blacked on both sides in order to absorb the maximum amount 

 of radiation. 



4. Otheoscope. — Inclined aluminium vanes driven by the mo- 

 lecular disturbance from the fixed black mica disk below, blowiug (so 

 to speak) through them. 



5. Otheoscope. — A large horizontal coloured disk of roasted mica, 

 driven by inclined aluminium vanes placed underneath it. 



6. Otheoscope. — A bright aluminium disk cut in segments, and 

 each segment turned at an angle, driven by a similar one below of 

 lampblacked silver. 



* Since writing this I have constructed such an instrument. The movement 

 takes place in the way I had anticipated.— W. 0., April 2Gth, 1877. 



