The Smithsonian Magazine's Naval Gazing: The Enigma of Étienne Bottineau provides a good overview. It was a brief craze on the scientific circuit at the time: obviously if it worked, it would have been of massive tactical advantage - an 18th century over-the-horizon radar. Google Books finds a number of contemporary descriptions, including the following sceptical one from Encyclopædia Britannica.
NAUSCOPY, the art of discovering the approach of ships or the neighbourhood of land at a considerable distance. This pretended art was invented by a M. Bottineau, employed in the King and Company's service in the island of France, from the year 1782 to 1784; and the account of it is given by the inventor as follows:
"This knowledge is not derived either from the undulation of the waves, or from the subtilty of sight, nor from any particular sensation; but merely from observation of the horizon, which discovers signs indicating the proximity of ships or of land
"On the approximation of a ship towards the land, or towards another ship, there appears in the atmosphere a meteor of a particular nature, visible to every one without any painful attention. It is not by any kind of accident that this meteor appears under these circumstances; on the contrary, it is the necessary result of the approximation of one vessel towards another, or towards the land. The existence of the meteor, and the knowledge of its different modifications, are what constitute the certainty and the precision of my informations.
"If I am asked, how it is possible that the approach of a ship towards land should give birth to any meteor whatsover in the atmosphere, and what connexion there can be between two objects at such a distance from each other? I reply, that I am not obliged to give an account of the hows and the wherefores: that it is sufficient for me to have discovered the fact, without being obliged to account for its principle."
The writer concludes, by desiring to be called on for experimental proofs, and by promising in future a complete treatise of Nauscopy, with maps, plates, &c.
This complete treatise, as far as we know, has not yet been published, nor do we expect ever to see such a treatise on the subject as will satisfy the minds of those who are persuaded that every effect must have an adequate cause. The administrators of the island, who gave to M. Bottineau what he calls a report, containing the most authentic and most explicit testimony of the reality of the discovery, seem to be of our opinion ; and yet they speak of this discovery with doubt, and with a degree of respect to which we think it can lay no claim. Their report is in the form of a letter directed to the Marechal de Castries: and that our infidelity may not deprive the public of what, inthe immense catalogue of possibilities, may lead to a useful discovery, we shall here subjoin a copy of it.
Port Louis, Island of France, the 18th February 1784.
"My Lord, A letter which you have written on the 6th of April to M. Bottineau, employed in the King and Company's service in this colony, obliges us not to refuse him one for you, os which he proposes being himself the bearer. The desire only of being useful to his country, is (as he says) the motive which determines him to take this step. He would be angry with himself were he to conceal a discovery which hath hitherto escaped the most enlightened persons, and of which he only is in possession. This, discovery is the art of announcing the presence of one or several ships, at 100, 150, and as far as 200 leagues distance. This is by no means the result of his studies, nor the fortunate application of the principles of any particular science; his science is in his eyes only, and he can have no other: what we call penetration and genius cannot make up to him what he is deficient in from education. He perceives (as he says) in nature, some signs which indicate to him the presence of the vessels, as we know that there is a fire in a place when we perceive the smoke which comes from it. This is the comparison which he .makes use of himself to those who have conversed with him about his art: this (though he has kept his secret to himself) is the plainest thing he has said, in order to make it be understood that he hath not made this discovery by the knowledge of any art or science, which had been the object of his application, or his former studies.
"It is according to him the effect of chance : he hath taken nature in the act, and hath discovered his secret ; so that his science, or rather the first elements of it, hath not cost him the least trouble; but the thing which hath cost him a great deal of labour, and which may be really called his own, is the art of judgeing of the exact distance.
"According to him, the signs very clearly indicate the presence of ships ; but none but those who can well read these signs can draw any conclusions from them with regard to distances; andthis art of reading them well, is according to him, a true and a very laborious study: for this reason he hath himself, for a very long time, been the dupe of his science. It is at least 15 years since he first foretold here the arrival of ships. At first this was regarded only as a frolic. Wagers were laid on both sides. He often lost, because the ships did not arrive at the time prescribed by him. From thence came his application to find out the cause of these mistakes ; and the perfection of his art is the result of this application.
"Since the war, his informations have greatly increased, and probably were sufficiently exact to excite the attention of the public. The noise of them reached us with the degree of enthusiasm which is always excited by the marvellous. He himself spoke of the reality of his science with a tone of a man convinced. It would have been too cruel to have dismissed him as a visionary.
"Besides, every thing depended upon proofs, and we required that he should produce some: in consequence, he hath regularly sent us, for eight months, the informations which he thought he might venture to send us; and the result is, that several of the ships he announced are arrived at the time he foretold, after several days of information.
"Others have come later than was expected, and some have not appeared at all.
"With regard to some of these, it hath been ascertained, that their delay had been occasioned by calms or by currents. M. Bottineau is persuaded, that those which never appeared were foreign vessels which went on; and accordingly we have learned, that some English ships were arrived in India, which might perhaps have been in sight of the island at the time indicated. But this is no more than a conjecture, which our occupations have not allowed us to investigate. What we can ascertain is, that in general it appears that M. Bottineau hath made just observations: whether it is owing to chance or to his abilities, it might be, perhaps, imprudent to determine. It is however certain, that the fact is so extraordinary, under whatever light it is.considered, that we have not thought ourselves able either to affirm or deny it; and we have wished the Sieur Bottineau to compel us to take one or the other side of the question, by trusting his secret to some trusty and able person. But this he hath refused, being probably afraid that he should not acquire by the discovery all the benefit which he imagines he may reap from it.
"Supposing the reality of the discovery, we do not believe that its utility can be as important as M. Bottineau persuades himself it is; but it might perhaps throw a great light upon natural history. In order to be useful it would be necessary that the discovery should be confined to one nation, and remain unknown to all others. This will be impossible, if every fleet, every vessel, and every privateer, is obliged to carry a man on board who is in possession of this secret.— We remain, with respect, my Lord, yours, &c. Le Vte. de Soulliac, Chevreau."
- "Nauscopy", pp 776-777, Encyclopædia Britannica: or, A dictionary of arts, sciences, and miscellaneous literature, Volume 12, 1797
In hindsight the evidence for the success of nauscopy looks heavily subject to confirmation bias. If ships arrived as predicted, nauscopy had worked. If they arrived later than predicted, there was the excuse that they'd been temporarily becalmed: nauscopy had worked. If they didn't arrive as predicted, they'd changed course (and would be identified with ships arriving elsewhere that - unprovably - might have been in the vicinity): nauscopy had worked!
Note that "meteor" doesn't have the modern meaning; in its older use, it applied to any kind of atmospheric phenomenon:
Atmospheric phenomena were formerly often classed as aerial meteor or airy meteors (winds), aqueous meteor or watery meteors (rain, snow, hail, dew, etc.), luminous meteors (the aurora, rainbow, sun halo, etc.), and igneous meteor or fiery meteors (lightning, shooting stars, etc.)
Bottineau's Extrait du mémoire de M. Bottineau sur la nauscopie (available in full via Google Books) postulates a mechanism for these "meteors":
The waters of the ocean form an immense gulf, in which in which substances of all kinds are swallowed up.Hmm.
The innumerable multitudes of animals, fish, birds, vegetable, and productions, which decay, and are decomposed in that vast basin, produce a fermentation abounding in spirits, salt, oil, sulphur, &c. &c. The existence of these is sufficiently apparent by the disagreeable smell and flavour of sea water, which can only be rendered drinkable by distillation and by the evaporation of those heterogeneous particles which infect it.
The spirits intimately united to the sea waters, continue undisturbed, a long as those waters remain in a state of tranquillity; or, at least, they experience only an internal agitation, which is slightly manifested externally.
But when the waters of the sea are set into motion by storms, or by the introduction of an active mass which rides upon their surface, with violence and rapidity, the volatile vapours contained in the bosom of the sea escape, and rise up a fine mist, which forms an atmosphere round the vessel.
This atmosphere advances with the vessel, and is increased every moment by fresh emanations rising from the bottom of the water.
These emanations appear like so many small clouds, which, joining each other, form a kind of sheet projecting forward, one extremity of which touches the ship, whilst the other advances into the sea, to a considerable distance.
But this train of vapours is not visible to the sight. It escapes observation by the transparency of its particles, and is confounded with the other fluids which compose the atmosphere.
But as soon as the vessel arrives within a circumference, where it meets with other homogeneous vapours, such as those which escape from land, this sheet, which till that time had been so limpid and subtil, is suddenly seen to acquire consistence and colour, by the mixture of the two opposite columns.
Tliis change begins at the prolonged extremities, which by their contact are united, and acquire a colour and strength; afterwards, in proportion te the progression of the vessel, the metamorphosis increases and reaches the centre. At last the phenomenon becomes the more manifest, and the shin makes its appearance.
- translation on page 351, The Monthly Anthology, and Boston Review, Volume 6, 1809