constantly going forward in the sun, and producing those radiations which are poured forth in volumes, far beyond the requirements of all the planets of our system. Although there is probably some circle of action analogous to that which exists upon this earth, maintaining the permanency of the vegetable and animal world, still there must be a waste of energy, which must be re-supplied to the sun.

May it not be, that Sir Isaac Newton's idea,-that the comets traversing space gather up the waste heat of the solar system, and eventually, falling into the sun, restore its power, is nearer the truth than the more modern hypothesis, that meteorites are incessantly raining an iron shower upon the solar surface, and by their mechanical impact reproducing the energy as constantly as it is expended.

THE PLYMOUTH BREAKWATER FORT.

BY S. J. MACKIE, C.E.

[PLATE LXXII.]

10 comprehend the nature and value of the Plymouth iron fort, its position, extent and nature should be clearly defined. Plymouth Sound is a vast open U-shaped natural harbour, a little over three miles in length, and about as wide at its mouth in a direct line between Penlee Point on the west, and Reny Point on the east. At about one-third of its area the turbulent waves which roll in from the Atlantic are checked by a great breakwater formed of rocks quarried from the neighbouring hills, mainly at Oreston. In length, the breakwater is about 5,000 feet long, rising up with very shelving slopes out of some six to seven fathoms of water (at low tide). On the eastern side of the Sound, Staddon Point juts out well towards the eastern arm of the breakwater; but on the western side the shore recedes between Penlee and Picklecombe into a large bay at Cawsand, forming a considerable expanse of water. The actual deep-draught shipways, of more than a thousand feet in width, pass on either hand close to the horns of the breakwater, nearly behind the centre of which, at a distance of a hundred yards, is placed the iron fort, to which this article is devoted. At Picklecombe and at Bovisand (the point of Staddon) there are granite casemated forts defending the water areas a little way in the rear; the distances of these batteries being each about a mile from it. A mile and a half up the Sound is Drake's Island, and half a mile further in the background the heights of Plymouth, also fortified, as well as are the entrances to the Hamoaze on the one hand, and to the Lairy on the other. The position of the breakwater fort is thus a most important one, and it is to be regarded as the main defence of the Sound.

The iron fort is based upon masonry foundations faced with ranite, and rising 16 feet 6 inches above high-water spring tides. Its walls are set in from the masonry about 3 feet, there being thus a clear walk or glacis of that width all round

it. In form it is an oval, measuring upon its major axis at the base 143 feet 6 inches, and upon the minor axis 113 feet 6 inches; the curves being struck from two radii, one of 90 feet 3 inches, the other 50 feet 9 inches. The batter of the wall is 1 in 11. In structure the iron wall consists of 15 inches of iron in three thicknesses, in the following order: -a front armour of three tiers of 5-inch plates, 21 feet 9 inches long, and respectively 4 feet 9 inches, 4 feet 1 inches, and 3 feet 5 inches wide (giving a total external height of 12 feet 5 inches); a backing of 6 inches of common concrete; next a layer of 5-inch bars 16 inches wide; these being crossed

behind by another layer of similar bars laid horizontally; the whole supported internally by vertical iron bars, 5 inches thick and 12 inches deep, placed upright and edgewise and let into the floor through a footplate, as also into the roof at short intervals. The entire circumference is pierced, at distances of 21 feet 9 inches from centre to centre, by eighteen ports for 10-inch 400-pounder rifled guns, which when in position will point variously-some seaward, some across each of the two shipways, and others up the Sound; but in no case does it appear that a converging fire of more than four guns can be obtained.

This brief view of the situation will show that the passage of any hostile fleet would have to be made along the contiguous