THE RADIOMETER. 
177 
It appears that up to a level position of gauge and barometer 
the viscosity of dry air does not vary much. On farther ex- 
haustion radiation becomes apparent, viscosity being unaltered. 
The latter then begins to diminish and the former to increase. 
At length, radiation approaches a maximum, but viscosity 
begins to diminish rather suddenly. When this last has sunk 
to one-fourth, radiation diminishes, reaching less than one-half 
of its maximum. The number of gaseous molecules present is 
then not practically infinite, and the path of each of them is no 
longer small as compared with the globe. This point of ex- 
haustion is far beyond that which is impervious to an induction 
current. 
The conclusions come to are — That the force is due only in- 
directly to radiation ; that the viscosity of the residual air is 
almost as great as at the atmospheric pressure ; that with other 
gases the results are similar in kind but different in degree ; 
and, lastly, that the repulsion is due to thermometric heat 
acting between the surface of the moving body and the case of 
the instrument. 
It may be best to notice at this stage some foreign contribu- 
tions to the history of the radiometer. As early as September, 
1875, a paper appeared in Poggendorf’s “ Annalen,” by Dr. F. 
Neesen, which advocated the influence of currents rising from 
inequalities of temperature. He used a modified form of ap- 
paratus, by which he established the fact that deflection dimi- 
nishes in proportion as vacuum is produced, and is at last 
supplanted by the opposite deflection. This he attributed to 
heat currents, and herein he was followed by M. Poggendorf 
himself in the subsequent November, who was of opinion that 
the motions would not take place in a perfect vacuum. The 
whole subject was, however, still in its infancy. 
In 1877 an excellent memoir was contributed to the“ Annales 
de Chimie et de Physique ” by MM. Bertin and Grarbe. The 
writers commence with a historical summary of experiments, 
adding to them an extremely ingenious modification of the 
instrument brought before the Academie des Sciences by Mons. 
Salet. The globe in this case contains two sets of vanes, one 
above the other, both of them fixed. Between them is a hori- 
zontal plate of mica turning on a point. Directly light strikes 
the apparatus, the mica disc begins to turn in the opposite di- 
rection from that which the vanes would assume if they were 
free to move. The plate of mica may be considered to repre- 
sent a portion of the case which has become movable, and 
therefore rotates just as the whole envelope does in Mr. Schuster’s 
experiments. 
A still more complete experiment, due to Signor Righi, is 
quoted from the “ Scienza Applicata,” Vol. I., part 2, page 8, in 
NEW SERIES, VOL. II. NO. VI. N 
