400 NAUSCOPIK. 



fresh emanations. These emanations are so many small particular 

 clouds, which by degrees joining each other, form a kind of cloud 

 (nappe), that projects forward, one extremity of which touches the 

 vessel, while the other extremity advances to a considerable distance. 

 This train of vapours is not on that account visible ; it escapes ob- 

 servation by the transparency of its parts, and it is lost among the 

 other fluids that compose the atmosphere : but as soon as the vessel 

 reaches a circumference, in which it meets with other homogeneous 

 vapours, such as those which escape from the earth, one perceives, on 

 a sudden, that cloud (nappe), until then so limpid and subtle, ac- 

 quire consistence and colour, by the mixing of the two opposed 

 columns. The change commences at the prolonged extremities, 

 which, by contact unite, are strengthened, and coloured ; and then, 

 every minute, as the vessel advances, the change is graduated, gains 

 the centre, and at length the engrainement being complete, the phe- 

 nomenon becomes more manifest, and the vessel appears. 



Such, in a few words, is the revelation of the cause and the effects 

 of a phenomenon, which, however wonderful it may be, accords, not- 

 withstanding, with physical notions. 



Whatever cause may be assigned for this phenomenon, it is quite 

 certain that it is the infallible satellite of a vessel ; and that, in con- 

 sequence of its prolonged form, it manifests itself to the eyes, one, 

 two, three, four, five, and even six days before the vessel itself, ac- 

 cording to the state of the weather, and the nature of the obstacles it 

 meets with. When the vessel sails with a fair wind (en poupe), and 

 meets with no obstacle, the phenomenon possesses its greatest cele- 

 rity; and, arriving sevaral days before the vessel, it affords the ob- 

 server the means of announcing the presence of a vessel at a consi- 

 derable distance ; but when the vessel meets with contrary winds, it 

 will be understood that this circumstance must have a great influence 

 on the progress of the phenomenon. On this account I state that 

 the phenomenon sometimes appears four or five days before the ves- 

 sel, and sometimes only one day. This defect of uniformity in the 

 apparition results from the greater or less impediment it meets with. 



It will naturally be supposed that there may be weather when the 

 phenomenon cannot show itself before the vessel : for instance, in a 

 violent gale, which appears, at first sight, capable of carrying away 

 the phenomenon even dissipating, and entirely destroying it. This, 

 however, is not the case. The most impetuous wind only retards the 

 apparition of the phenomenon, without destroying it. But when the 

 vessel has reached a certain distance from land, then the phenomenon 

 has acquired so much consistence, that it overcomes the efforts of the 

 strongest winds, which, though they agitate it, still leave some parts 

 which they cannot wholly disperse. 



The whole of my science consists in being able to follow the ap- 

 parition of this meteor, and distinguish its character, in order not to 

 confound it with other clouds in the atmosphere, and which are not 

 to be attended to. In order to make these observations, neither 

 telescopes nor mathematical instruments are required ; the eyes alone 

 are sufficient. 



It is not even necessary to be upon the coast ; where the horizon 



