COMMUNICATION ON HEAT BETWEEN A SUEEACE AND A GAS. 
733 
sphere is that of the area of a great circle to that of the sphere, or J ; so that it is pro- 
bable the photometer only registers -f the difference of temperature which similar surfaces 
would acquire on the mill. 
The white surfaces on the mill, however, are not similar to those of the photometer, 
and they probably absorb considerably more light, and consequently diminish the 
difference of temperature ; so that, on the whole, it is probable that the differences 
recorded by the photometer are quite as great, if not greater than those which exist in 
the mill. 
The instrument is very sensitive, and begins to move as soon as the light falls on it. 
Its indications agree surprisingly with those of the light-mill : 1° on the photometer 
corresponds with 11 revolutions per minute of the mill. When the mill made 200 
revolutions per minute, the reading on the photometer was 21°, which is the highest it 
will record. Differences to of a degree can be read on the photometer, or the 
effect of light which will turn the mill at 1 revolution per minute. It can be used, 
therefore, for all purposes of photometry for which the mill may be useful. It is much 
more convenient, as it requires no counting, and it can be made with much less 
trouble. 
Measured by this photometer, the difference of temperature in Dr. Schuster’s experi- 
ment would have been 24°. This, which must be looked on as an outside measure, leaves 
ample room for allowance for the inaccuracy of the calculation. We have, on the one 
hand, the least estimated heat l 0- 7, and the greatest limit of the measured heat 24°, and 
the probability that both these quantities tend towards each other. 
Conclusion. 
The investigation of which this paper gives an account was undertaken with a view 
to settle the only point respecting my previous explanation of the motion caused by heat 
which appeared to me to remain doubtful, after I had discovered that, according to the 
kinetic theory, the communication of heat to a gas was attended by a reaction on the 
surface, viz. whether this reaction was adequate in amount to produce the motion. This 
point has now been cleared up. We have : — 
1. The remarkable agreement between the law of the resistance experienced by the 
mill and the peculiar law of the resistance which air offers at small tension. 
2. Dr. Schuster’s positive proof that the force which acts on the vanes arises within 
the mill itself. 
3. The exceedingly small magnitude of the actual force, as shown by quantitive 
measurements. 
4. The fact that the estimated difference of temperature necessary to produce heat- 
reactions of equal magnitude with the forces which act is well within the difference of 
temperature actually found to exist. 
Taking all these facts into consideration, it seems to me that the evidence is conclu- 
sive as regards the nature of the forces which cause the motion in light-mills, and that 
mdccclxxvi. 5 i 
