122 THE EFFECT OF WAVES UPON A TAFFRAIL LOG. 
noticed. As the mechanical construction of the log seemed open to sus- 
picion it was most carefully rebuilt in the shop of the Institute and then 
tried out on the progressive speed trials of the United States Revenue Cutter 
Gresham over the Government course at Provincetownin1g10. ‘The results 
of these trials showed such an erratic behavior of the log constant that it 
led the author to suspect that another source of trouble than any of those 
previously mentioned was the real cause; namely, the effect of the waves 
which the ship carries along with itself. 
Leaving the results of these trials for a moment let us consider the 
effect upon such a log that might be expected from the waves maintained 
by the ship. 
Figure 2 shows the profile of a trochoidal wave traveling across the page 
from right to left. This wave is composed of particles each rotating in a 
circular orbit and the orbits of three of these particles are shown, one at 
the hollow, one at half way from hollow to crest and one at the crest. Now 
Trochoidal Ware 
Direclion of 
Propogation 
rfose when atrest 
Small arrows indicate direction of molien of particle 
Fic. 2. era 
if a log were to be dragged in this wave at the same speed that the wave 
was going and from a line of such length that it came at a hollow, we should 
expect that the rotator would be spun faster than it would be in still water, as 
the particles are moving away from the ship. In other words we should 
expect the log to overrun in its readings and the factor by which it would 
‘be necessary to multiply the log reading to get correct speed would be less 
than what it would be in still water. Conversely if the log were towed on a 
crest this factor would be larger than when in still water and half way be- 
tween (the middle particle in Fig. 2) there would be no effect, as the par- 
ticles are moving at right angles to the log and should not affect it. 
All ships when under way maintain two systems of waves, a bow system 
and a stern system each consisting of diagonal waves on each side with trans- 
verse waves which terminate in these diagonal waves as indicated in Fig. 3. 
These transverse waves are apparently nearly trochoidal waves and consider- 
ing only the stern wave system, we should expect to find a log which was 
towed behind a ship, alternately overrunning and underrunning as the ship 
