SHIPs WAY. 165 
The greateft difficulty, perhaps, will be in graduating 
the arch K; (if the machine is conftructed as in figure 1,) 
the unequal divifions of which can only be afcertained by 
actual experiment on board of each fhip refpeCtively; in 
as much as the accuracy of thefe graduations will depend 
on three circumflances, viz. The pofition of the fulcrum 
C with refpect to the length of the rod, the fize of the palate 
D and the ftrength or bearing of the {pring L. When thefe 
graduations, however, are once afcertained for the machine 
on board of any one veflel, they will not want any future 
alterations; provided the palate D be kept clean, and the 
{pring L retains its elafticity. : 
But the unequal divifions of the graduated arch will be 
unneceflary, if the machine is conftructed as in figure 4. 
For as the chain goes round the barrel L, and then winds 
through the fpiral channel of the fuzee W, the force of 
the main-fpring muft operate equally, or nearly fo, in all 
pofitions of the index; and confequently, the divifions of 
the arch K may, in fuch cafe, be equal. 
After all, it is not expected that a fhip’s longitude can 
be determined to a mathematical certainty by this inftru- 
ment. ‘The irregular motions and impulfes to which a 
fhip is continually expofed, make fuch an accuracy unat~ 
tainable perhaps by any machinery: But if it fhould be 
found, as I flatter myfelf it will on fair experiment, that it 
anfwers the purpofe much better than the common log, it 
_may be confidered as an acquifition to the art of navigation. 
It fhould be obferved that in afcertaining a fhip’s longi=- 
tude by a time-piece, this great inconvenience occurs, that 
a {mall and trifling miftake in the time, makes a very great 
and dangerous error in the diftance run: Whereas the er- 
rors of this machine will operate no farther than their real 
amount; which can never be great or dangerous, if cor- 
rected by the ufual obfervations made by mariners for cor-= 
recting the common log. 
A like machine made in its fimple form, (as at fig. 1.) 
‘fo conftructed as to ihip and unfhip, might occafionally 
x be 
