68 THE TROCHOIDED PLANE. 
of the ripples are transversal to the longitudinal axis, and move 
towards the tail ; the slime is a great aid in passing over a smooth 
surface, as a partial vacuum can be formed in the hollow of each 
wave; if the surface of the slug’s foot is covered with cilia, the 
undulatory motion of these may be conversely seen on a field o 
waving corn as the breeze passes over it ; the mode of progression 
of some caterpillars is parallel with that of the slugs. 
In order to make this more than mere theory, it became neces- 
sary to pursue one of two courses ; either to go in for an elaborate 
system of instantaneous photography in connection with a chrono- 
to make some models, the geometrical construction of 
which would show the trochoided plane, and the outward appear- 
ance and movement of the apparatus would appeal direct to the 
eye. I have adopted the latter course, for several reasons, the 
principal one being that the first method, besides being very 
expensive to me, would only be accessible, if understood at all, by 
the few ; whilst the second course is now within the reach of any 
boy who can handle a few tools. As these are experiments that I 
venture to call capital, it will be excusable if the details seem 
rather trivial. 
The simplest trochoided plane may be constructed by attaching 
a flat surface at right angles to the connecting-rod of the ordinary 
crank and connecting-rod motion of the reciprocatin, engine ; an 
if two of these be coupled with the cranks at right angles (Fig. 1), 
the sum of the sectional areas of the columns of wind or water, 
act on the planes at right angles to the guides, a uniform rotary 
motion will be communicated to the crank-shaft ; if we rotate the 
machine by steam or hand, motion is communicated to the air or 
