MR. W. CROOKES ON REPULSION RESULTING FROM RADIATION. 
303 
This apparatus consists of a wide glass tube, a a, b b, drawn off narrow at the end 
b b, and a stem, c, sealed in to hold a needle point. To the narrow end a fine tube, d, 
is attached to connect the apparatus to the mercury pump. Round the needle is 
placed a ring of fine platinum wire, e e, the ends of which are joined to thicker 
Fig. 30. 
platinum wires passing through the glass at ff. A current of electricity from two 
Grove’s cells, turned on or off by a contact key, gives the power of making the wire 
ring, e e, red hot when desired. The top of the wide tube is ground and polished 
quite flat, and is covered by a piece of plate glass, g, which can be cemented on so as 
to form a perfectly tight joint, and may be removed by warming so as to admit of any 
experimental fly being supported on the needle point. 
The fly used in these experiments consists of four square vanes of thin clear mica, 
supported on light aluminium arms, and having in the centre a small glass cap, which 
rests on the needle point. The vanes are inclined at an angle of 45° to the horizontal 
plane. They are in such a position that when rotating the centres of the vanes pass 
along the platinum ring and keep about 5 millims. distant from it. 
337. In describing the direction of rotation of this fly, I shall consider the observer’s 
eye to be on a level with the plane in which the fly rotates, and the direction recorded 
will be that taken by each vane as it passes in front. Assuming that the fly shown 
in fig. 30 is rotating in the same direction as the hands of a watch, were the watch 
laid face upwards on the top of the plate glass cover, each vane will be fore-shortened, 
and passing the observer will have the appearance of /. The direction of rotation 
in this case will be expressed by / , and will be considered as 'positive rotation 
- — ix., as the direction which would be followed by the fly were molecular pressure 
to proceed from the platinum wire. 
338. The apparatus was exhausted till the gauge showed a pressure of 8 millionths of 
an atmosphere. Contact was made with the battery, the platinum ring became hot, 
