Night Signals.
The system of occulting light applies with remarkable facility to night signals, either on shore or at sea. If it is used numerically, it applies to all the great dictionaries of the various maritime nations. I may here remark, that there exist means by which all such signals may, if necessary, be communicated in cipher.
Sun Signals.
The distance at which such signals can be rendered visible exceeds that of any other class of signals by means of light. During the Irish Trigonometrical Survey, a mountain in Scotland was observed, with an angular instrument from a station in Ireland, at the distance of 108 miles. This was accomplished by stationing a party on the summit of the mountain in Scotland with a looking-glass of about a foot square, directing the sun’s image to the opposite station. No occultations were used; but if the mirror had been larger, and occultation employed, messages might have been sent, and the time of residence upon the mountain considerably diminished. When I was occupied with occulting signals, I made this widely known. I afterwards communicated the plan, during a visit to Paris, to many of my friends in that capital, and, by request, to the Minister of Marine.
I have observed in the “Comptes Rendus” that the system has to a certain extent been since used in the south of Algeria, where, during eight months of the year, the sun is generally unobscured by clouds as long as it is above the horizon. I have not, however, noticed in those communications to the Institute any reference to my own previous publication.