Vol. 8, 1922 
PHYSICS: A. G. WEBSTER 
105 
the spiral (which may be judged from the fact that for 6 = 45°, a = 23 °.2' ; 
p = 22.35 compared with p = 22.28 for the spiral, and that the rear 
wheel's track is less than two feet distant at the maximum) it is not here 
reproduced. 
It may be said that the above is not applicable to the way in which an 
automobile actually skids, when it is well known that the two wheels do 
not break the frictional constraint together, but the rear wheel generally 
skids first. This ma}^ be true, but I think this will not invalidate anything 
that I have said. One would suppose that this question would be treated 
in any elementary book on statics, but as I have failed to find it in any 
treatise, I insert a treatment here. It will now be necessary to consider 
a third path, namely, that of the center of gravity of the machine, which 
we shall still assume to be on the line joining the wheels, at distances d and «i' 
therefrom, figure 3. Then we have d'ctna'^ = (d d')ctna. The three 
O 
FIG. 3 
paths have the same center of curvature. The constraints due to the 
friction can be only at right angles to the paths of the wheels, and we will 
call them F, F'. The kinetic reaction of the machine is described by the 
centrifugal force C, the tangential reaction R, and the moment 5. It 
may be asked how the wheels can cause any reaction in the path, but it 
will be seen that there must be such when we consider that energy of 
translation being converted into energy of rotation and conversely will 
give rise to acceleration in the path, negative or positive, respectively. 
In fact we see at once that 5 must vanish with R, for three concurrent 
