Mr. TV". Crookas on the Otheoscope. 69 



perfect in many respects. The black or driving surface, correspond- 

 ing to the heater of the engine, being also part of the moving fly, 

 is restricted as to weight, material, and area of surface. It must 

 be of the lightest possible construction, or 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. Again, the part 

 corresponding 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. 



A perfect instrument would be one in which the heater was sta- 

 tionary; it might then be of the most suitable material, of sufficient 

 area of .surface, and of the most efficient shape, irrespective of weight. 

 The cooler should be the part which moves ; it should be as close as 

 possible to the heater, and of the best size, shape, and weight for 

 utilizing the force impinging on it. By having the driving surface 

 of large size, and making it of a good conductor 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 were blowing from 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 like an actual wind. 



TVhilst 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 (wflew, 

 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 requsite 

 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 fly will not move, however strong 

 the incident radiation. But place the otheoscope in the same con- 

 ditions, and it will move as well without the case as with it. 



In the preliminary note already referred tot, I described a piece of 

 apparatus by which I was able to measure the thickness of the laver 

 of molecular pressure generated when radiation impinged on a 



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

 ference of air from one place to another. This molecular movement may be 

 compared to the movement of the gases when water is decomposd by an electric 

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

 although eight times as much matter is passing one way as the other. 



t Proc. Royal Soc. Nov. 16, 1876, p. 310. 



