At the eighth year they begin to yield interest on the capital and cost of keeping up (which is almost nothing), from the combustible that is obtained by removing the surplus, from making charcoal, &c. &c. At twenty years they begin to extract resin from the trees, at thirty years the produce is most abundant, and continues up to eighty years, when the wood is fit for building purposes. Then new seed plots are formed in proportion as the old trees are removed. The annual produce of resin gathered from a _hectare_ of pines is an average of five _metric quintals_, which, at the rate of twenty-two _francs_ per quintal, gives a revenue of 110 _francs_, and represents at least seventy per cent. of the capital employed. It will be seen by this statement, that the fixing of the downs has become one of the most productive operations of Silviculture, and that it offers one of the most advantageous investments for capital. It was therefore necessary for us to know, whether the downs which cover the northern part of the Isthmus could be fixed by the same process. Now, we have ascertained, 1st. That the greater part of these downs are naturally fixed by a multitude of different plants, which have found sufficient moisture for their support; 2nd. That the moveable downs of the basin of Lake Timsah conceal moisture at a very little depth below their surface; they may, therefore, be fixed by seed plots, and to do it there is the immense quantity of bushes and shrubs which grow in the low parts surrounding the lake, and which give to that region the appearance of a copse. Not only will sufficient branches be found on the spot for the tufted seed plots, but moreover, all the combustibles for the lime, and for the wants of the workmen, will be furnished abundantly. It is true, that at the time we made our observations (January, 1855), tolerably abundant rains had recently moistened the soil; the success of the seed-plots, which would be destitute of moisture for the rest of the year, might therefore be doubted; but what is there to prevent the moist season being chosen for making these plots? and the seed once risen, the abundant moisture which is felt in this region, especially during the summer nights, will suffice to support vegetation, as is seen by the downs fixed naturally. Finally, to remove all objections, the fresh water canal which will end at Lake Timsah, will supply, if required, the means of affording, during the early days of the seed-plots, sufficient moisture for the success of the undertaking.