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By: Jean-Claude Sabrier.

Pierre Le Roy's

Pierre Le Roy (Paris1717–Vitry1785)

And his 'Mémoire sur la meilleure manière de mesurer le temps en mer', 1770.

Table of contents:

Some Assumptions.
Sully's escapement enhanced.
The first proper example.
Trials at Sea.
Four other watches of this type are known.
No. 4732.
No. 4757.
Le Roy invenit, Laresche renovavit.
'Les Montres de Pierre le Roy', Sabrier's extensive source paper in French.

Use 'F11' to view full screen.


Some assumptions influencing the construction of his pocket watches  “for the use of astronomers and seamen” were expounded by Pierre Le Roy for the first time in an article appended in his:

Mémoire sur la meilleure manière de mesurer le temps en mer, (published in 1770).


...they will never be as perfect because of their small size which does not allow us that employ all the devices for reduction of friction etc… that we incorporate in our chronometers.

I believe however, that they may be made more accurate:

1) by giving a more perfect isochronism to the balance

by employing compensation for  the effects of heat and cold, similar to that used by Mr Harrison in his timekeeper.

by using a dead beat escapement where the friction is a lot less than in a cylinder, etc. By this I do not mean a detent escapement, for the practical application of which, a watch is, I believe, too small...."


Fig. 1
Sully's escapement.


...The escapement of Mr. Sully where the wheel is perpendicular to the plates, having been perfected, seems to me the best way of achieving this reduction in friction.

With this last point in mind I have made several watches, giving the escape wheel a size such that it go as nearly to the dial on one side and nearly to the balance spring on the other. I have also a given the teeth the form of spokes to make the wheel very light, and in conjunction with certain other improvements I believe it may now be shown that because of the reduction of friction obtained by the locking of the wheel very close to the balance staff, this is the best escapement of all.


The first proper example of this type was made to the order of Monseigneur le Duc de Penthièvre , Grand Admiral de France. It was apparently the only one to be submitted to the judged by the Académie Royale des Sciences.

It is also in this paper that he gives precise details of his motives for constructing the first watch of this type and are reprinted and, more importantly, put in order in Mémoire sur une nouvelle montre à l’usage des astronomes et des marins, read to the Academy on the 3rd July 1771.  


....I call them Seamen's watches and not Marine chronometers, because they take the place of an ordinary pocket watch. When employing them for use at sea, they are placed in their suspension, somewhat similar to those I employ for my marine chronometers, in order that they are maintained in the same position and that position being the most propitious, will enable them to go with even greater accuracy. In effect, when the watch is flat, the balance is carried by the pivot ends where the friction is a lot less than along the sides and in addition, I have arranged the escapement in such a way, that in this position, the action of the escape wheel relieves the weight of the balance. This arrangement together with the isochronism of the vibrations of the regulator, achieved by the method published in my memoir, the considerable reduction in friction This arrangement, together with the isochronism of the vibrations of the regulator, achieved by the method published in my memoir, the considerable reduction in friction. However, the new feature, which is very significant, is the compensation for the effects of heat and cold. I mentioned in my appendix Mr Harrison's which would be easy to apply to this English artist's timekeeper but certainly not to a pocket watch; especially when one considers that in regulating the length of the balance spring, a strength is required which is not consistent with the thin and narrow blades of metal that the space in a watch would appear to allow. Besides, these blades may only be very short so how is it possible for them to exert sufficient effect on the balance spring. I found the solution:

1) by making the blades work, not directly on the spring itself, as in the timekeeper, but through a lever which makes the effect five times more sensitive.

2) b
y arranging the wheels in such a way that there is sufficient space left to fit the thermometer, by which I mean the blades. Their length is almost equal to the diameter of the watch and their height is almost that of the frame of the movement and they thus have a strength that they would otherwise not have.

Consequently the thermometer has another very important advantage and one found wanting in that of Mr Harrison, which is to provide a simple means of increasing or decreasing the desired effect, by means of a screw which adjusts the proximity of the metal blades to the centre of the lever.....

In order to place the compensation blades between the two plates, Pierre Le Roy had to make the wheel train without fusee.

Fig. 3. (click to enlarge)
This first watch, which unfortunately has not been found, is relatively well known through the drawings that accompany his memoir


On board of the frigate La Flore, during the trials with marine timekeepers “A” and “S”, the watch was known as the Petite Ronde because of the shape of the box that contains it.


To day, four other watches of this type are known to exist. Apparently the second one to have been made is now in the Beyer Museum of Time Measurement, in Zurich.

Fig. 5 (click to enlarge)
Almost certainly the second 'Petite ronde' 
ade by Pierre Le Roy

Fig. 7 (click to enlarge)

Fig. 9 (click to enlarge)

Fig. 11 (click to enlarge) end

On this watch, which is apparently the second one of the type, to have been made, the compensation blades are set on the back plate in order to leave enough room between the plates for a fusee. In this arrangement however, the bimetallic compensation is not adjustable.

End of this section, click here to continue.

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Pierre Le Roy
fig. 2 (click to enlarge)
Mémoire sur une Nouvelle montre
à l'usage des Astronomes et des Marins.
(go to transcription)

NO. 4732

Fig. 13 (click to enlarge)

Le Roy No. 4732 appears to be he third watch of this type known to have been made. It is almost identical to that in the Beyer Museum, but the temperature compensation device, on the back plate is adjustable.

Fig. 15 (click to enlarge)
Le Roy 4536, Petite Ronde with adjustable
compensation blades on the back plate.

NO. 4757

The two last watches of this type known to exist are in the collection of the Patek Philippe Museum in Geneva.

One of them is certainly the first ever made in France with a detached lever escapement. Il carries the serial number 4757 and its temperature compensation, has been constructed in England by George Daniels; in a time the two other Petites Rondes were not discovered yet, there for it was made after that, fitted on the back plate of the watch by Ferdinand Berthoud, No 417, which then was the sole to be known with a temperature compensation.

Fig. 17.

Fig. 19 (click to enlarge)

Fig. 21.

Fig. 23.

Fig. 23 (click to enlarge)

Pierre Le Roy 4757, the lever escapement with roller pallets - Patek Philippe Museum

Its lever with roller pallets was extremely heavy and therefore it never worked, due to its inertia.


Also made as an experimental piece, the other one, was specially conceived to be used in a vertical position. Therefore, its balance was set into the cage, perpendicularly to the plates. It is a double face watch and its movement was found without dials, nor case. It was judged of sufficient importance to be restored by Henri Lasreche in 1804, with a very fine silver case with gold rims, and two white enamel dials with hours and minutes on the front face, the seconds on the reverse,  the central hand, revolving anti clockwise.

The front face carries the inscription: Julien Le Roy Invenit, Paris 1772, H. Laresche  Renovavit Paris 1804,  on the reverse.

Fig. 27.

Fig. 29.

The Montre de marins, as it was named by Pierre Le Roy in his Mémoir, as opposed to his two celebrated Montres Marines.   Patek Philippe Museum - Genève

Fig. 31.

The movement fitted in a ring, in order to enlarge its diameter.

The fact it was the first marine timekeeper ever made to be carried in a pocket, is certainly the main interest of this watch.

Julien Le Roy (1686-1759) Horloger.
Julien Le Roy

The dial is inscribed: Julien Le Roy Invenit 1772, in order to commemorate the date of its invention by Pierre Leroy, who almost allways signed his pieces with the name of his father after his death in order to glorify his memory.

Jean-Claude Sabrier. 2010

Editor: W.A. van Klaveren.  Producer: F. Kats.

Webmaster's note:
This article is subject to ongoing revisions.


End of article.