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XLII. Memoir on the Usefulness of Time-Keepers in the 
Service of the Navy; and a Plan for introducing them 
with the best Prospect of Success to ensure their Accuracy 
at the least Expense. Communicated ly Mr. FirM1nGer, 
late Assistant Astronomer at the Royal Observatory, 
Greenwich *, 
N OTWITHSTANDING the great advantages derived to our 
navigation from the improved, and it may be said almost 
perfected state of the science of astronomy, aided by equal 
improvements in the construction of instruments employed 
in its application, by which the determination of the longi- 
tude has been reduced to a problem of no difficulty to per- 
Sons possessing even but a moderate acquaintance with the 
mathematics ; yet we still find the method of determining 
the situation of a ship at sea by the lunar observations 
considered perhaps by the greater part of nautical men a 
laborious and difficult undertaking, and, when performed, 
subject to many mistakes,— mistakes which the mariner 
from his habits of lite will be more liable to make than 
most other men. And although no endeavour should be 
wanting in urging the necessity towards a perpetual atten 
tion to the lunar observations; yet it need not prevent our 
having recourse to other means that can in any respect 
simplify, or render more easy or certain, the acquirement of 
this important object. 
The advantage of time-keepers to navigation is so ge- 
nerally felt by commanders of ships destined to long voy- 
ages, that few if any of the commanders of vessels em- 
ployed in the service of the honourable East India Company 
do not take out two or more of these machines. ~ 
The lunar observations, when taken and reduced with 
care, will give the longitude at sea with an uncertainty 
seldom exceeding twenty or thirty miles, a distance which 
perhaps will be allowed in all cases sufficiently exact for 
the practical purposes of the mariner: and which can 
never subject him to dangers he is not prepared to meet. 
But it often happens that theée most truly useful observa- 
tions, from various causes, cannot be put in practice, and 
an interval of eight or ten days and sometimes a month 
may elapse before an opportunity shall occur for the ma- 
riner to avail himself of their aid: here the use of a good 
time-keeper will become highly advantageous. The time 
* This memoir was written at the express wish of an individual, and was 
_ presented to the Admiralcy, but whether they had ever any intention of in- 
zroducing such a plan the Author does not know: 
Vol, 42, No, 186, Oct. 1813, Q at 
