113 
Sea Log and Sounding Machine. 
solutely necessary, that this first mover should be out of 
the wake of the vessel, it had a length of fifty yards of 
cord, or more, to carry round with it every time it revolv- 
ed. The friction caused by this operation was such as to 
preclude ail hopes of accuracy; it was useless in an agi- 
tated sea, the rope was very liable to kink, and in fast 
sailing the rotator would sometimes fly out of the water. 
These circumstances rendered it impossible, that the ro- 
tator should make the same number of revolutions in pass- 
ing through a given space, under different velocities ; and 
hence inaccuracy was unavoidable. To get rid of this 
friction of the long line, the rotator has, in some instances, 
been enclosed in a cylinder, and a register been attached 
to the outside. But though the defect of excessive fric- 
tion was thus surmounted, still greater inconveniences re 
suited. It may be sufficient to mention, that the cylin- 
der, not presenting itself horizontally in the water, was 
liable to alter its position whenever the velocity of the ves- 
sel was changed, which caused an eddy, or dead water, 
to remain in the cylinder; and, of course, the rotator was 
liable to err, in proportion as the cylinder lost its horizon- 
tal position. 
After thus hinting at the imperfections of other pre- 
vious methods of constructing logs, it remains to point out 
wherein Massey’s plan differs. 
Friction is the principle cause of mechanical theories 
varying so widely from actual experiment. In some ma- 
chines one third is allowed for its effect, while the opera- 
tion in others is nearly suspended, and what appeared 
* Smeaton, in the account of his experiments on Saumarez.’s log, in the Philo- 
sophical Transactions, observes on this subject : ei Upon making up the account 
of this run, I found the number of rotations were less by one full third than they 
ought to have been, compared with the former observations, which afforded me 
a convincing proof, that this instrument was considerably retarded in quick mo- 
lions.” 
VOL. IT, 
O 
