The weight of wood is of great importance, because its hardness, resistance, and its heating power, as well as other valuable properties, are all more or less depending upon it. In the first place, we must consider that even wood which has been forested very light will become heavy, when put for some time into water, but in such timbers the sap is already given to dissolution. If the fibre were the only substance in the wood, then the specific weight would depend upon the number of pores contained in its body; the pores are, however, filled with a substance such as resin, die, &c. Some years since, when the Indian railways were being formed, the native wood-cutters were so well aware of the above-mentioned fact, that they used to cut down the soft and inferior woods in the forests; soak them in water for a certain time; and then endeavour to pass them to the railway contractors as sound, heavy, and good railway sleepers, and the latter, not being acquainted with the Indian woods, were, at first, often deceived. The hardest, and heaviest woods come from the hotter climates; the only exception is the pine, which thrives considerably better, and furnishes heavier timber, when it has grown in colder regions, or upon high mountains. Trees grown on northern slopes furnish lighter timber than if grown on southern or western. The soil has great influence upon the width of the yearly rings, and from this we are able to come to a conclusion in regard to the specific weight. In the fir and larch trees the wood is heaviest when their rings are smallest. The difference in the strength of timber between the south and the north side is attributable to the grain being closer on the north side, as the sap does not rise in the same proportion as upon the south. In forest-grown wood the difference is almost imperceptible, as the sun cannot act upon the trunk of the tree; in open-grown timber, the difference is really perceptible. It is well known that all woods do not lose strength by being open grown, or, in other words, that the south side is not always weaker than the north; that theory only applies to the coniferæ species. In ash it is the opposite, as the south side is the strongest. In soft-wooded trees, as the acer species, the difference is not perceptible, as the annual rings, and the intervening cellular tissues, are so close akin as to render the wood so compact in its grain that there is no difference in its strength. The coniferæ species, or the pines, are the only classes of woods that are stronger on the north side than on the south: it is well known that the difference originates in the wood being more open in the grain on the south side than on the north.