wheel put on, and then a second piece of cork; the end of the pin was then stuck into a quill or a pencil, and thus was formed an apparatus very like a child's windmill, except that it had two sets of vanes, each revolving in opposite directions. On walking across a room towards a window, or a candle, with this little toy in the hand, or blowing at it slightly from the mouth, the lines were beautifully seen, being either stationary or moving, according to the relative velocity of the two wheels. This could be altered at pleasure by inclining the vanes more or less, or by blowing towards the centre of the wheels, or towards the edges when the larger hind wheel received more propulsive force.
Spinners or whirligigs formed of discs of cardboard stuck upon pins, and upon which radii, either straight or curved, or other forms, had been drawn in bold lines with black ink, when spun upon a sheet of paper, and then looked at through the moving fingers or through equidistant bars of pasteboard moved before them, show a great many of the effects.
Finally, a couple of open radial wheels (fig. 1) upon pins or wires, if revolved between the fingers in different positions and directions, show a great many of these effects extremely well. Their shadows may be thrown ppon each other, or upon the wall; one may be held near the eye, when it acts like a grate with parallel bars; and if one side of each wheel is black whilst the other is white, still greater variety may be obtained. They will be quite sufficient, when employed in a few experiments, to make anything in this description clear, which I may have left obscure.
The curious appearance exhibited by the wheel animalcule has such resemblance to some of those described in this paper, that they inevitably associate in the mind of a person who has witnessed both effects. This little insect has been well described by Mr. Baker[1] and others, and can only be viewed distinctly under a high magnifying power; it then presents an elongated sack-like form (fig. 17), either attached by the posterior part to the side of the vessel containing the water in which it exists, or else floating in the fluid. When the effect in question is observable, there is seen the appearance of two wheels, one on
- ↑ Baker on the Microscope, vol. ii. p. 266; see also Leeuwenhoek, Phil. Trans., Nos. 283, 295, 337; and Adams on the Microscope, p. 548.
