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[1 r] Mountpleasant near Philadelphia October 29. 1792.

Sir.

I beleive I have had the Honor of seeing you at the House of my late venerable Relation Dr Franklin when he was minister plenipotentiary at the Court of France; But whether you recollect me or not, I trust the Subject of the inclosed Memoir will plead its own Apology.

The Minister of France here has kindly undertaken to transmit a Copy to the marine Department of your Nation, and through the same medium I take the Liberty of addressing one to you for <L’Academie> L’accademie des Sciences a Paris.

It has hitherto received no other publicity than a Reading before the Philosophical Society here, and a place in their third Volume of Transactions now in the press. The few Copies I have had Struck off being merrely to save the^{1Ce mot a été écrit deux fois.} Trouble of multiplying the manuscript in this, & some other Communications.

The Efforts of Genius for a Discovery of the Longitude have hitherto me[e]t with great Difficulty in practice, owing to the alterations all Time Instruments [1 v] are^{2Réclame à la page précédente.} Subject to by being exposed alternately to a dry, a moist, a warm & a cold atmosphere, and the Inaccurary of Celestial Observation from a Surface always in motion. But in the present Case, neither Genius nor Science are requisite to keep a thermometrical Journal of the Temperature of the Sea; and an approach <will> to the Coast will be known at a Distance sufficient to enable a Navigator to avoid Danger. Where the Extent of such Coast bas been previously ascertained in a Direction towards the Ocean, he may correct his Reckoning as compleatly as if the Land were in Sight. And this might be done with equal Case & Certainty, in the most boisterous and the most moderate Weather.

I do not presume to state any theoretic principles on this Subject, nor do I ask Confidence in what may appear matter of assertion only; But I should be happy (for the Love I bear to the Cause of Humanity) that the Plan were Supported by repeated Experiments [2 r] To^{3Réclame à la page précédente.} Persons accustomed to thermometrical Observations it would be needless to give any Directions relative to a Communication of Heat from any foreign Cause; but for general practice it may not be amiss to observe.

1. The Bucket to draw the water should be of Wood and kept solely for that use.

2. It should tow alongside about ten minutes, to give it the Temperature of the Sea.

3. The Water should be taken from the Ships Stern where it is more mixed than ahead or alongside.

4. The Instrument should be put into the Bucket the Instant it is drawn up, & the degree should be noted as soon as the mercury becomes Stationary before the Ball of the Instrument be taken out of the Water.

5. Every Experiment Should be repeated before it is registered.

6. In the night time a Lanthern should be used, otherwise the light might affect the Instrument.

7. All Experiments should be made out of the Wind: If the Instrument when out be held in the Air, the Evaporation will make it change in a moment: It is for this Reason that the Degree [2 v] Should^{4Réclame à la page précédente.} be noted before the Ball of the Instrument be taken of the Water.

8. The Temperature of the Air should be first noticed when the Thermometer is dry; this should be done out of the Wind & in the Shade.

9. There should be several Instruments provided to supply accidents.

I beg you Sir to submit this Letter with one Copy of my memoir to The Accademy, & <[... ?]> |hope you will| do me the Honor to accept me for your <[... ?]> particular Use.

I am with great Respect Sir Your most obedient & most humble Servant

Jon.a Williams^{5Paraphe bouclé.}

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