which is then exhausted to the highest attainable point and hermetically sealed.
The arms of this instrument rotate with more or less velocity under the action of radiation, the rapidity of revolution being directly proportional to the intensity of the incident rays. Placed in the sun,
Figs. 7 and 8.—a, a very fine needle-point; b, two pieces of straw; c, jewel-cup; d, d, d, d, four pith-disks, blackened on one side. The arms between the straw in the centre and the disks are bent glass-fibres; e, glass support holding cup; f, cement to keep the support c in its place.
or exposed to the light of burning magnesium, the rapidity is so great that the separate disks are lost in a circle of light. Exposed to a candle twenty inches off another instrument gave one revolution in 182 seconds; with the same candle placed at a distance of ten inches off the result is one revolution in 45 seconds; and at five inches off one revolution was given in 11 seconds. Thus it is seen that the mechanical action of radiation is inversely proportional to the square of the distance. At the same distance two candles give exactly double, and three candles give three times the velocity given by one candle, and
