THE KADIOMETER. 
179 
greatest sensitiveness is obtained. Eotating instruments will 
not move to moonlight, but a sensitive torsion balance does so. 
The writer adopts Mr. Johnstone Stoney’s explanation as given 
above, which has been tested by rotating the same fly in two bulbs 
of different size. An ingenious instrument has been made with 
a large and small bulb containing a single fly which can be 
balanced in either of them, its vanes being distant \ inch from 
the walls of one and J inch from those of the other. In the 
smaller bulb the fly rotates about 50 per cent, faster than in the 
large. 
The chief novelty, however, in this paper is the influence of 
the shape of the surfaces on the rotation. If the aluminium 
plates latterly used as vanes were turned up at the corners so 
as to make the blackened surface concave, it was attracted instead 
of being repelled by a candle, though still repelled by dark 
heat. Sloping the plates of a lamp-blacked mica radiometer, 
so as to have the black outside, and more facing the side of the 
bulb, greatly increases the sensitiveness. 
Cup-shaped and conical discs materially modify the action 
and require more experimental investigation. A convex surface 
seems to cause greater pressure between itself and the bounding 
surface than one which is concave. Many illustrations of these 
facts are given in the paper, which want of space alone forbids 
our reproducing. 
We thus arrive at the last in the long series of Mr. Crookes’s 
papers, a series which his immense energy and unparalleled 
industry render it laborious even to chronicle. This was read 
on April 26, 1877, before the Eoyal Society. He therein 
describes a new form of instrument for which he suggests the 
title of Otheoscope (from mOsco, I propel). 
In the radiometer, the surface which produces molecular dis- 
turbance is mounted on a fly, and is driven backwards by the 
excess of pressure between it and the sides of the containing 
vessel. As a heat-engine this arrangement is imperfect. The 
heater should be stationary, and the cooler movable. The 
driving surface should be of large size, and a good conductor of 
heat. The blackened surface acts as if a 66 molecular wind ” were 
blowing from it, principally in a direction normal to the surface. 
This wind blows away whatever movable body happens to be in 
front of it, irrespective of colour, shape, or material; in its 
capability of deflection from one surface to another, its arrest 
by solid bodies, and its tangential action, it behaves in most 
respects like an actual wind. 
The otheoscope, founded on these data, may be made similar 
to the radiometer, or in different forms. In the former case, at 
■one side of the bulb is fastened a plate of mica blackened on one 
side, in a vertical plane. The vanes are of clear mica, and 
