Page:Passages from the Life of a Philosopher.djvu/479

Rh which they are submitted at sea. If, therefore, after a run of above two hundred miles, they can be informed of the exact Greenwich time, the sea rate of their chronometers will be obtained.

Of course no other difficulty than that of expense occurs in transmitting Greenwich time by electricity to any points on our coast. The real difficulty is to convey it to the passing vessels. The firing of a cannon at certain fixed hours has been proposed, but this plan is encumbered by requiring the knowledge of the distance of the vessel from the gun, and also from the variation of the velocity of the transmission of sound under various circumstances.

During the night the flash arising from ignited gunpowder might be employed. But this, in case of rain or other atmospheric circumstances, might be impeded. The best plan for night-signals would be to have an occulting light, which might be that of the lighthouse itself or another specially reserved for the purpose.

During the day, and when the sun is shining, the time might be transmitted by the occultations of reflected solar light, which would be seen at any distance the curvature of the earth admitted.

The application of my Zenith Light might perhaps fulfil all the required conditions during daylight.

I have found that, even in the atmosphere of London, an opening only five inches square can be distinctly seen, and its occultations counted by the naked eye at the distance of a quarter of a mile. If the side of the opening were double the former, then the light transmitted to the eye would be four times as great, and the occultations might be observed at the distance of one mile.

The looking-glass employed must have its side nearly in