N° XI. 
Defcription of a Machine for Meafuring a Ship’s Way 
through the Sea, by F. Hopkinson, E/quire. 
pect HE errors and uncertainties incident to the 
3 mariner’s log, in common ufe, are too obvious 
not to be univerfally acknowledged. Were it not for the 
obfervations navigators are accuftomed to make of a fhip’s 
progrefs, by the apparent paflage of the water along fide, 
and the figns that ufually prefent on approaching the land, 
the log, alone, would be a very unfafe dependence. 
Several attempts have, therefore, been made to improve 
the log, and render its indications more accurate, but with- 
out fuccefs. All the machines of this kind, that I have 
heard of, were compofed of a number of wheels, which 
were to be put in motion by the twifting of a line let out 
aftern, having a drag at the end fo conftructed as to whirl 
round, fafter or flower, according to the motion of the 
fhip. 
The objections to a machine, fo conftructed, are, Firft, 
If the line is not very long the drag will be confiderably 
affected by the fhip’s wake; and, if it is very long, the 
twiftings will be irregular, and the line liable to kink. 
~ Secondly, Vf the drag is fo heavy as to fink below the bot- 
toms of the waves, when the fhip fails faft, it will be too 
heavy, and fink too deep when fhe hath but a flow pro- 
greflive motion: Or, otherwife, if the drag is of a proper 
weight when fhe makes but little way, it will {kip from 
wave to wave through the air when fhe fails with a brifk 
gale, and fo be of noufe. And, Thirdly, It will be liable 
to moft of the other irregularities to which the log in 
common ufe is expofed. 
2 ‘Eke 
