p for common uses.
It is clinker-built, from 12 to 14 feet long, and has a beam one third of its length.
An ancient altitude, angle, and leveling instrument; said to have been invented by Hipparchus.
The dioptric system of lighting, used in lighthouses, as distinguished from the catoptric, which is by reflectors.
Refraction instead of reflection.
Lenses were used in the South Foreland light in 1752, and in the Portland light, England, in 1789.
The system fell into disfavor, owing to certain mechanical difficulties in the construction and arrangement of the lenses.
It was revived and improved by Fresnel about 1810, and has been generally adopted throughout France and Holland, and partially in England.
It is considered superior to the catoptric, and was readopted in England in 1834, being placed in the Lundy Island Lighthouse, Devonshire, England.
The Fresnel dioptric lamp consists of a mechanical, four-wicked oil-lamp, plac