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POPULAR SCIENCE REVIEW. 
which, after repeating the flotation and suspensory contrivances, 
the radiometer is floated on water upside down, so that its vanes 
rest on the projecting piece intended to prevent their falling off 
the needle point. In this case they form one piece with the 
case itself. The strongest beam from the oxyhydrogen lamp 
then directed upon them fails to produce the slightest deviation. 
Upon these data a mathematical investigation of the forces 
involved in the rotation is entered into, in which the compara- 
tive weights of the case and the vanes, previously ascertained, 
are considered with reference to the combined motion of the 
two. It was found, that with a radiometer suspended in air, 
the currents due to its heating by the ray of light interfered 
with the strict ratio of mobility. The instrument was there- 
fore suspended under the receiver of an air-pump, and the 
pressure reduced to 3 millimetres of mercury. Experiments 
made under these conditions agreed well with one another. 
Tables of these are annexed. The ratio of the vanes’ speed to 
that of the case was always a little too great, as if the latter 
had been slightly retarded by the torsion of the suspension and 
the residual air of the receiver. 
A memoir by Professor Challis, contributed to the April 
number of the “ Philosophical Magazine,” upon the cup-shaped 
radiometer of Mr. Crookes, with both sides bright, can only be 
briefly named for lack of space. The author attributes the 
rotation to thermo-electric setherial currents, and appears to 
deny the presence of residual gas as the cause of motion. 
On November 16, 1876, Mr. Crookes brought another series 
of experiments before the Royal Society, stating that he had 
succeeded in experimenting at still higher exhaustions, and with 
different gases. By means of a McLeod gauge he had been 
able to measure the pressure at any desired stage. He had thus 
estimated repulsion and viscosity, plotting the observations in 
curves, showing the relation of the two to one another. These 
are given with the paper. 
Some technical improvements, such as the use of phosphoric 
anhydride instead of monohydrated sulphuric acid for drying 
the gases enable the rarefaction to be pushed to a higher degree 
than before. In this way, and sometimes with the assistance of 
chemical absorption, it is not difficult to exhaust a radiometer 
to such a point that it will not move to a candle placed a few 
inches off ; but it has not been possible to stop the movement 
of the beam in the torsion apparatus. 
The improvements added by Mr. Grimingham to the mercury 
pump render it so easy to obtain high exhaustions, that in 
preparing radiometers he now exhausts to one or two millionths 
of an atmosphere, then uses a hot-air bath of 300° C. to drive 
off occluded gases, and then attenuated air is let in until the 
