Yet they are undoubtedly the result of the very slow cooling and contraction of melted rocks under compression by strata below and above them, so that, when once solidified, the mass was held in position and the tension produced by contraction could only be relieved by numerous very small cracks at short distances from each other in every direction, resulting in five, six, or seven-sided polygons, with sides only a few inches long.

The curve of marginal convenience in audibility would be founded upon actual 'polygons of variation' recording measurements of the distance at which a sufficient number of individuals of the classes and ages expected could hear and make themselves heard in a room of that shape.

If we treat the general aptitude (or 'cleverness') of the children as differing only by more or less, the problem becomes one of fitting the types of school to a fairly exactly ascertainable polygon of intellectual variation.

" She adds: "Perhaps you may be interested in the following account from my sister of her visual peculiarities: 'When I think of Wednesday I see a kind of oval flat wash of yellow emerald green; for Tuesday, a gray sky colour; for Thursday, a brown-red irregular polygon; and a dull yellow smudge for Friday.'

Fixed in the floor of the tank are 144 posts, 9 inches thick at the bottom and 6 inches thick at the top, to support the crown of the holder in such a way that the tops are fixed in a kind of socket, each of them being provided with four horizontal bars, which decrease in thickness from 305 by 100 mm. to 150 by 50 mm., and represent 16 parallel polygons, which in their turn are fastened diagonally by means of iron rails 63 by 100 mm.

Rare and far, the sails of junks patched the horizon with umber polygons.