2095. The uneven surface of sheet-glass has been supposed the principal cause of the scorching of the leaves of plants grown under it. In June, 1848, Mr. Mitchell, gardener to II. Willyams, Esq., of Carnanton, finding that his plants were scorched very irregularly, examined the glass of his houses, and found that in some places the glass was much thicker than in others, so that these parts of the glass acted as a burning lens upon the leaves of the plants. 'This effect,' Dr. Lindley observes, 'arises from the peculiar mode in which sheet-glass is manufactured. When sheet-glass is made, the first operation is to form a cylinder, which is afterwards slit on one side, and spread into a flat sheet. Now, if we assume a cylinder, of 26 oz. glass, to have its sides 0.125 of an inch thick, the inner circumference will be &&& of an inch less than the outer. But when the cylinder is spread into a flat sheet, the two surfaces become of an equal width, the glass adjusting itself by the expansion of the inner or smaller surface, and by the contraction of the outer or larger surface. In this operation are formed what the manufacturers call 'cockles,' producing that uneven puckered appearance which is the peculiar characteristic of sheet-glass; and of these cockles some are circular, and form lenses of considerable power.' From the preceding observations, it is quite clear that the use of sheet-glass, whatever may be its colour, will always be attended with some risk of injuring the plants grown under it; and the reason why it burns in some places and in others does not, is also evident, as 'all depends on whether the leaves come within the foci of the lenses, or whether such lenses exist in the squares, some squares having lenses, and others not,' (Gard. Chron. for 1848, p. 539.)
