Page:Popular Science Monthly Volume 24.djvu/65

Rh longer possible to count. This instance may be taken to illustrate how unsatisfactory this mode of reckoning really is for all but trees of comparatively youthful age.

The external girth measurement is for these reasons the best we can have, being especially applicable where the date of a tree's introduction into a country or of its planting is definitely fixed, since it enables us to argue from the individual specimen or from a number of specimens, not with certainty, but within certain limits of variability, to the rate of growth of that tree as a species. In these measurements of trees of a century or more in age, such as are given abundantly in Loudon's "Arboretum," lies our best guide, though even then the growth in subsequent ages must remain matter of conjecture. The difficulty is to reduce this conjectural quantity to the limits of probability; for, given the ascertained growth of the first century, how shall we estimate the diminished growth of later centuries? The best way would seem to be to take the ascertained growth of the first century, and then to make, say, the third of it the average growth of every century. Thus, if we were to take twelve feet as the ascertained growth of an oak in its first century, four feet would be its constant average rate, and we might conjecture that an oak of forty feet was about a thousand years old. But clearly it might be much less; for the reason for taking the third is not so much that it is a more probable average than the half, as that it is obviously less likely to err on the side of excess of rapidity.

The cypress affords an instance where the approximate certainty of its introduction into England enables us to form some conclusions with regard to its attainable age. The fact of its being first mentioned in Turner's "Names of Herbs," published in 1548, makes it probable that it was not introduced into England before the beginning of that century. But, at all events, the cypress at Fulham, which in 1793 was two feet five inches at three feet from the ground, can not have been planted there before 1674, the year that Compton, the great introducer of foreign trees into England in the seventeenth century, became Bishop of London. That gives a growth of about two feet in the first century; but sometimes it attains a higher rate, as in the case of the cypress planted by Michael Angelo at Chartreux, which was thirteen feet round in 1817, giving the average rate of over four feet in the first three centuries. Now, the cypress at Somma, between Lake Maggiore and Milan, for whose sake Napoleon bent the road out of the straight line, is not more than twenty-three feet in girth, so that the tradition which makes its planting coeval with Christianity would seem doubtful; though if we take three feet as the first century's growth, and take the third as the average, it may evidently have been standing in the time of Cæsar, as an old chronicle of Milan is averred to attest.

The Lebanon cedar first planted at Lambeth in 1683 was only seven