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His radiometer will turn in the light of day and in the darkness of the night; it will turn everywhere where there is heat, and heat is everywhere. But, unfortunately, this beautiful little machine, while it goes down to posterity as the most interesting, must likewise be put on record as the most inefficient machine ever invented!
M. Lebedeff a few years ago allowed a sheaf of rays from an arc lamp to fall on a deflection radiometer, and thus succeeded in revealing the existence of this pressure. Its value is sufficient, in the case of matter of little density and finely divided, to reduce and even change into repulsion the attractive action exercised on bodies by the sun.
Instead of using the fan in this experiment, we may use the "electric" radiometer with similar effect. But in this case it will be found that the vanes will rotate only at high exhaustion or at ordinary pressures; they will not rotate at moderate pressures, when the air is highly conducting. This curious observation was made conjointly by Professor Crookes and myself.
Perhaps the greatest wonders of the 19th century were the invention of photography, solar spectral analysis, the radiometer, the phonograph, the photophone; in public affairs, the reunion of the old and new school Presbyterian Churches, and the disappearance, by civil war, of negro slavery in the United States.
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