The celebrated experimental philosopher, Sir John Leslie, was the inventor of this instrument, the purpose of which is to give a comparative idea of the radiation proceeding from the surface of the earth towards the sky. It consists, as represented in fig. 66, of two glass bulbs united by a vertical glass tube, of so fine a bore that a little coloured liquid is supported in it by its own adhesion, there being air confined in each of the bulbs. The bulb, A, is enclosed in a highly polished brass sphere, D, made in halves and screwed together. The bulb, B, is blackened and placed in the centre of a metallic cup, C, which is well gilt on the inside, and which may be covered by a top, F. The brass coverings defend both bulbs from solar radiation, or any adventitious source of heat. When the top is on, the liquid remains at zero of the scale. On removing the top and presenting the instrument to a clear sky, either by night or by day, the bulb, B, is cooled by terrestrial radiation, while the bulb, A, retains the temperature of the air. The air confined in B, therefore, contracts; and the elasticity of that within A forces the liquid up the tube, to a height proportionate to the intensity of the radiation. Such is the sensitiveness of the instrument, that the smallest cloud passing over it checks the rise of the liquid. Sir John Leslie says:—“Under a clear blue sky, the æthrioscope will sometimes indicate a cold of fifty millesimal degrees; yet, on other days, when the air seems equally bright, the effect is hardly 30°.” This anomaly, according to Dr. Tyndall, is simply due to the difference in the quantity of aqueous vapour present in the atmosphere. The presence of invisible vapour intercepts the radiation from the æthrioscope, while its absence opens a door for the escape of this radiation into space.
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