40 Le Roy’s Memoir on the best Method 
hausted, the air is readmitted by a stop-cock under the plate 
of the receiver. 
If you think the above improvement worth inserting in 
your Philosophical Magazine, by so doing you will greatly 
oblige i 
WW arestens Your very humble servant, | 
Sept. 12, 1806. T. SYLVESTER, 
IX. A Memoir on the best Method of meusuring Time at 
Sea, which obtained the double Prize adjudged by the 
Royal Academy of Sciences; containing the Description 
of the Longitude Watth presented to His Majesty* the 
5thof August 1766. By M.Lx Roy, Clock-maker to the 
King t. Translated from the French by Mr.T.S. Evans, 
E.L.S., of the Royal Militury Academy, Woolwich. 
A crear deal has lately been said on the subject of chro- 
nometers, more especially with regard to what is contained 
in the description given by M, le Roy of his time-keeper ; 
and the work to which that description is subjoined being 
now in the hands of very few persons, the translator thought 
this paper in English might be a valuable addition to the very 
little useful matter which we possess on that branch of me- 
chanics. It has been asserted that the greater part of the 
improvements in chronometers, lately laid before the Board 
of Longitude, are mentioned in this account of Le Roy’s ; 
the public will therefore now have it in their power to judge 
for themselves. Tate Las 
— 
Labor omnia vincit 
Improbus, Virg. Georg. lib. i. 
INTRODUCTION. 
By proposing to determine the lest method of measuring 
time at sea, the Academy appears to require, first, such a 
measure of time as may give to mariners the knowledge of 
the longitude which has been so much desired, that for many 
* Louis XV. king of France, 
+ This Memoir is subjoined to the Voyage fait par ordre du Roi en 1768, 
pour prouver les montres marines inventées par-M. Le Roy, par M. Cassini, fils. 
Paris 1770, 4to. 
ages 
