THE EFFECT OF WAVES UPON A TAFFRAIL 106. ip 
where— 
H=wave height in feet. 
c =speed of wave crest in feet per second. 
\=wave length in feet. 
v=speed of particle in feet per second. 
and if we encountered waves whose ratio of length to height was about 
40, then— 
ns 2 =on0 85 ¢ 
40 -0705 
or about 8 per cent. of the speed of the wave crest. A log towed at the sur- 
face of such a wave would vary that amount above and below the normal 
according as it was on a crest or hollow, and less as its immersion increased. 
The results of these trials show definitely: (1) That any taffrail log 
is seriously affected by the wave system carried along by the ship. (2) 
That in order to be available for measurement of distances at different 
speeds it must be calibrated by obtaining many points at closely adjacent 
speeds, and in some manner that takes account of the wave action. (3) 
The length of the log line must be unchanged throughout calibration and 
subsequent running; and (4) that a taffrail log as a close measurer of speeds 
is a questionable instrument. We occasionally find captains at sea who have 
faith in their logs and who obtain excellent results from them after they 
have rated them, and this may be quite possible if the vessel sails always 
at the same speed and the log is towed from a constant length log line, but 
if a log which is correctly rated for one ship be taken into another ship 
which has a different length or speed it will usually be far from correct. 
TABLE IV.—U. S. R. C. GRESHAM. 
Dimensions of Hull, Engines, and Propeller. 
Wererthwovenyalllte. rec crs Me ee eae oe mies 205 ft., 6 in. 
Length between perpendiculars............ 188 ft., o in. 
Vromldedubeaite ses vk cogs kos ces Ree: 32 ft., o in. 
Wasnt EAN a. 2c aoe) ec es ees eels Bt, 1s tia. 
Wiearikclirahteortls (italia 6 tae cust eo ity tea es T2. it... Oniit: 
Displacement at tral.drait... 2. ..5...2.55 1,024 tons. 
NV etee cist cee wisi cio eve0y State) s Seeap ste aeye is. tes 7,330 sq. ft. 
Diameter high-pressure cylinder........... 25 in. 
Diameter intermediate cylinder............ 275 int: 
