Aethrioscope, an instrument invented by Sir John Leslie for measuring the relative degrees of cold produced by the radiation toward a clear sky.In a metallic cup standing upon a tall hollow pedestal, a differential thermometer is placed in such a manner that one of its bulbs is in the focus of the paraboloid formed by the cavity of the cup, and the other bulb is beyond the hollow of the cup. The interior of the cup is highly polished, and is kept covered by a plate of metal, and only opened when an observation is to be made. As the second bulb is out of the cup, it is not affected by the radiation, the action of which is concentrated upon the first bulb. The contraction of the air in this bulb by its sudden exposure to a clear sky causes the liquid in the stem to rise. The figure represents a vertical section of the Aethrioscope.