556 



Messrs. G. J. Stoney and R. J. Moss [Feb. 22, 



Sprengel pump. The smaller tube terminates in a contraction bearing a 

 stopcock which serves for admitting the gases to be experimented 

 upon. 



We found it necessary to avoid the irregular actions which arose 

 when the incident light was allowed to shine on the inside of the glass 

 tube. This was accomplished by projecting on the disk the image of a 

 uniformly illuminated circular aperture in a screen of copper foil 

 placed outside the glass chimney of an Argand gas-burner. The lens 

 employed for this purpose is permanently attached to a stand on which 

 the lamp is secured. When the position of the pith disk is altered, the 

 position of the stand carrying the lamp and lens is altered to the same 

 extent, so that the pith disk is always in focuso The burner is automa- 

 tically supplied with coal-gas at the uniform rate of 3*2 cubic feet per 

 hour, this being the quantity that gives a flame of the required size. 



We found that the torsion of a cocoon fibre furnishes a force which is 

 too variable to admit of its being delicately controlled by the method 

 just referred to ; but a very accurate adjustment was secured by a sup- 

 plementary arrangement. It has already been mentioned that the arm 

 which bears the thin glass disk carries a small iron weight by which its 

 balance is regulated. This weight was made to serve for balancing the 

 torsion of the silk fibre. Tor this purpose a small bar magnet sliding in 

 a groove is so placed that one pole acts on the weight. With a little 

 care the distance of the magnet from the weight can be adjusted so as to 

 bring the index to zero, and thus exactly counterbalance the torsion of 

 the silk, the index remaining practically stationary. In this condition 

 the apparatus is sensitive to an extreme degree. 



It will be observed that in this apparatus the cooler of the heat- 

 engine consists of the swinging disk along with that part of the contain- 

 ing tube which lies bet\^'een the swinging disk and the disk of blackened 

 pith. By thus making a portion of the cooler freely movable, we hoped 

 to be able to ascertain the thickness of the layer of gas within which 

 Crookes's force exists. It would not have answered for this part of our 

 investigation to have made the heater the part freely movable, as in all 

 apparatus of the kind that had been previously constructed, because the 

 heater cannot be placed far from the cooler in apparatus that is not in- 

 conveniently large for the Sprengel pump, since when the containing 

 tube is of any moderate size its sides become the principal part of the 

 cooler* when the glass disk is at a distance. 



* It is obvious, from the dynamical theory, that if the molecules tending in one 

 direction within a stationary gas are at one temperature, while the rest of the mole- 

 cules of the gas and the surface of a solid with which they come in contact are at 

 another temperature, then the Crookes's force which arises may be either normal to 

 that surface like the pressure of a gas, or tangential to it like friction, or in any way 

 compounded of these two, being in each case in the direction spoken of above. 



Accordingly the forces that act upon the containing vessel and the vanes of radio- 

 meters are in general partly tangential and partly normal ; so that in estimating the 



