. Chambers's encyclopedia; a dictionary of universal knowledge for the people. tirthwith a velocity of from 3 to 9 miles an hour, andcauses numerous eddies and whirlpools. It is esti-mated that about 4000 vessels with cargoes passthrough the Firth annually. PENTLAND HILLS, in the Lowlands ofScotland, extend north-east from the border ofLanarkshire to the centre of the county of ivlin-burch, and to within 4 miles of the city of thatnan?e. Mean height upwards of 1000 feet; highestsummit. East Cairn, near the middle ot the range,1839 feet. PENUMBRA. When the shadow of an opaqueobiect is thrown up
. Chambers's encyclopedia; a dictionary of universal knowledge for the people. tirthwith a velocity of from 3 to 9 miles an hour, andcauses numerous eddies and whirlpools. It is esti-mated that about 4000 vessels with cargoes passthrough the Firth annually. PENTLAND HILLS, in the Lowlands ofScotland, extend north-east from the border ofLanarkshire to the centre of the county of ivlin-burch, and to within 4 miles of the city of thatnan?e. Mean height upwards of 1000 feet; highestsummit. East Cairn, near the middle ot the range,1839 feet. PENUMBRA. When the shadow of an opaqueobiect is thrown upon a surface at some little dis-tance by a li^ht of considerable apparent size, it isobserved that the shadow is divided into twoportions, a dark portion in the centre, an. a lighterportion it The is known i«ihe umhra, or complete shadow ; the latter as thepenumbra, or partial shadow. A reference^ the PENZA—PEPR figure will at once make plain their origin andrelation; for if S be the illuminating body, Ethe objent whose shadow is east on the surface,. ABCD, it is seen that the small portion, uu, receives(omitting all consideration of refraction, disper-sion, &c., of light) no light from S, while the wholesurface outside of PPPF is completely point Preceives light from the whole of S ; thepoint F is only half illumined, and that by thelower part of S, the illumination of the pointsbecoming less and less as they approach u\ which isunillumined. The portion within uu is the umbra,and that between the boundaries PPPP and uu isthe penumbra, which, as we have seen, graduallyshades from jjerfect light at the outer boundary toperfect darkness at the inner, so that it is almostimpossible exactly to note its limits on either phenomenon, it is evident, can only occurwhen the illuminating body is of such a size, real orapparent, as to make the angle, PKw., of sensiblemagnitude; and it is equally evident that thenearer the body E approaches t
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