70 Royal Society : — 



of glass 4 millims. in thickness ; through this light is admitted to 

 the pith disk by an arrangement to be presently referred to. The 

 other end of the large tube is contracted and terminates in a nar- 

 row tube bent upwards, partly packed with gold leaf (to intercept 

 mercury vapour), and attached to the exhaust-tube of a 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 per- 

 manently 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 automatically 

 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 supplementary 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. For 

 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 containing tube which lies between 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 mthin 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 inconveniently 

 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 



