MR. D. WIXSTANLEY ON THE RADIOGRAPH. 87 



pendulous oscillations, notwithstanding the presence of 

 the liquids it contains ; for these remain substantially at 

 rest whilst the tube which holds them does^ in fact, slide 

 over them (and with very little friction too) in swinging 

 to and fro through arcs o£ the circle of which its parts 

 are curves. Both bulbs of the thermometer are closed. 

 It is obvious therefore that the tension of the air or gas 

 which they contain will be uninfluenced by the barometric 

 variations of the outer air, the temperature of which latter 

 being experienced equally in each bulb will also leave the 

 equilibrium of the apparatus undisturbed. When, how- 

 ever, one bulb is more heated than the other, the air con- 

 tained therein will press more strongly on the heavy 

 liquid piston in the tube and wheel the swinging portion 

 of the system round until a fresh position of equilibrium 

 is gained, and this will be (providing that the centre of 

 gi'avity of the system has previously been made coincident 

 with the point on which it turns) when the tension of the 

 gases is equal in both bulbs. In fact, in so far as now 

 described, the instrument is a differential thermometer, 

 and is that alone — differing in this from Leslie^s, that it is 

 a solid and accessible portion of the thing which moves 

 and not the liquid it contains. When, however, one of the 

 bulbs is blackened and the other one is silvered or left 

 clear, the apparatus becomes a " radiometer " in the 

 proper meaning of the term* — that is to say, a measurer of 

 the thermal radiance to which it is exposed and the inten- 

 sity of which it indicates by variations in the angular posi- 

 tions of a needle prolonged from one or other of the radii 

 of the wheel. 



It is only needful now so to arrange it that this needle 



* The "radiometer" of Crookes should in its simple form have been 

 called a " radioscope," as it merely makes visible the effects of radiance, but 

 does not measure their amount. 



