Imatges de pÓgina




m) 5 ft

CHAP. I.-Influence of Woods on the Temperature of Countries. Woods lower the temperature. In the author's opinion they have this effect, because, on account of their dark colour, they reflect back few rays of light and heat to the atmosphere, and because they keep the soil damp, and therefore render more heat necessary to promote evaporation. But whether the difference of the mean temperature between Rome and Massachusetts, which, in the same geographical latitude, amounts to 63 cents of a degree, is to be attributed almost entirely to the woods, does not appear so easily demonstrable. That woods on mountains have a great influence in intercepting and beating down vapours is very probable; but the author's assertion that the increased condensation of vapours has for its consequence a greater absorption of heat, is unsupported by proof. It must also be observed, that though woods diminish temperature, and though the author proves this fact by practical observations, yet the experiments as cited by him are so detached, as to leave room for considerable objection. Vienna and Troyes have not a difference of temperature, merely because eastern countries are generally more covered with wood, but also because the site of Vienna is 250 feet higher, and lies nearer high mountains than Troyes. Again, 'Berlin is colder than Leyden in Holland, not altogether in consequence of Brandenburg being a woody country, bat also because it is exposed to the cold east wind. The translator points out the difference of height, which in the original is unattended to.


[ocr errors]

The author shews from historical comparisons, that the clearing away of woods makes the temperature of countries. warmer. Whether the proofs that he adduces for this purpose are all quite convincing, we cannot positively affirm, though the 'fact itself is true; for France is no more deprived of its woods than England. If, then, there be, at present, nearly the same "degree of heat in London and Paris, while, in the time of Tacitus, Gaul must have been colder, it is not quite clear why this 'contrary influence should proceed from the same cause. But "it is also doubtful whether Tacitus would not even now, if he

had no assistance from the thermometer, call London milder Lorie di do bur-zu941-45 74106


than Paris, because severe winters occur less often in the former than in the latter city. But though the details of these comparisons may occasionally be uncertain, and more particularly those which relate to the question, whether a difference of temperature in former times ought to be attributed solely to the great abundance of woods and forests, yet a collection of the accounts of a greater degree of cold having heretofore existed, is doubtless very important. There can be but few to whom it will not be interesting to learn how much the cold of winter, especially in the south of Europe, has diminished.

CHAP. II.—Influence on the quantity of Rain.

The author here brings forward some observations to prove that more rain falls on the sea-coast than in inland districts, and that, moreover, when chains of mountains run parallel to the sea-shore, the sides next the sea receive more rain than their opposite sides. In reference to woods, however, the author supposes that he may lay it down as a fact confirmed by observation that wood-lands in flat countries do not perceptibly increase the quantity of rain; but that woods on mountains have a perceptible influence in producing that effect. He thereupon founds the conclusion, that if mountains are planted with trees, the quantity of rain in their vicinity will be increased, and that the progressive diminution of rain in the South of Europe is to be ascribed to the destruction of the mountain woods. But the author does not appear to have been sufficiently careful in ascertaining what are the places in which the rain has diminished. It has not diminished in all parts of the south of Europe; for Flaugergues has found that the quantity of rain has considerably increased at Viviers, in the south of France, since 1778. The Milanese Ephemerides indicate the same thing for Milan, and all the assertions of this kind require further demonstration.

[ocr errors]
[ocr errors]

CHAP, III. Of the Influence of Woods on the Humidity of the Atmosphere.

Here the author describes among other things an interesting experiment with the hygrometer, according to which the humidity of the air in the West Indies is found to be expressed by

the numbers 3, 4, and 15, according as the observations are made on the coast in the midst of cultivated plantations, on the borders of mountain woods, or in the midst of those woods. There seems, however, to be some obscurity in an observation which the author is induced to make, namely, that the humidity of woods in the torrid zone extends far above the extremity of the scale. Now, as the hygrometer is usually graduated up to the point of complete humidity of the atmosphere, it can only be said that the moisture is precipitated in greater abundance than is necessary to bring the hygrometer to the highest degree.

We cannot venture to quote any more of these observations, or to explain the grounds of the doubts to which the conclusions drawn by the author give rise. But notwithstanding these doubts, we confess with pleasure that much information is to be found in the work. At the same time we must regret that the translator has not given to his version that great superiority over the original which it would have obtained had he subjected several of the author's statements, such as those relative to the comparative humidity of the Mark of Brandenburg and Holland, to critical investigation.

[ocr errors]

CHAP. IV.—Influence of Woods on Springs and Running Water.

That countries, especially mountainous countries, which are covered with woods, also abound more in waters than others, is a fact which may be asserted with little fear of contradiction.

CHAP. V. Of the Influence of Woods on the Wind and on the State of the Atmosphere with respect to Health.

Though many remarks which occur in this chapter are just, we are much surprised at some of the assertions, and we think we do not err in considering them unfounded. Among these is the assertion that the impetuosity of the winds, where there are no woods to mitigate its violence, has rendered a great part of Great Britain barren. If the author's estimate, that the waste lands amount to of the whole surface of Great Britain be true, it does not follow that these lands are barren in consequence of a deficiency of trees. The heaths in the north of Germany, which are not all situated in places entirely destitute

of woods, shew clearly enough that circumstances, which accompany an effect, are not always those which produce it.

The comparison between that part of Tartary inhabited by the Calmucks and Lombardy, appears to be equally unfounded. Whoever in this instance, though the geographical latitude should be the same, expects to find the climate in both regions alike, and ascribes the dissimilarity of the climate to the want of trees in Tartary, must certainly have allowed many circumstances, which ought to have been taken into account, to pass unnoticed.

CHAP. VI.—Influence of Woods on the Fertility of the Soil. We also meet with many remarkable observations and important conclusions in this chapter. But upon the whole, we think that this essay must be considered as a work which has not been reflected upon with sufficient deliberation. Nevertheless, it contains an abundant collection of curious facts; and though some of these facts are not well applied, and the accuracy of others remains to be proved, the book will, in the mean time be found an excellent contribution towards the explanation of the subjects of which it treats.

There are occasionally some obscurities in the reasoning; but whether these ought to be attributed to the author or the translator, we have not at present the means of determining.

Additional Remarks on the Stowage of Ships. By Commander JOHN PEARSE, R. N.

[ocr errors]

My former remarks were written with the intention of endeavouring to shew that we have no regular system for stowing our ships, and that an alteration in the distribution of the weights is necessary. Subsequent to their publication, I have seen an authentic calculation of the weight and pressure, according to the ordinary distribution in a modern 74-gun ship. This has induced me to publish these additional remarks, by which I shall be enabled to prove that the alterations I proposed would materially contribute to remedy the inequality of the weight and pressure represented in the calculation alluded to; and which is referred to in the following observations of Dr.

Young's, in his remarks on the employment of oblique riders. Philos. Trans. 1814.

"It is unnecessary to explain here the well-known inequality of the distribution of the weight and pressure, which causes almost all ships to have a tendency to arch or hog, that is, to become convex upwards, in the direction of their length. It is possible that there may be cases in which a strain of a very different nature is produced; but in ships of war this tendency appears to be universal.

"It is, however, very different in degree in the different parts of a ship; and, of course, still more different according to the different modes of distribution of the ballast and stores, which may occur in different ships; but, in ordinary cases, it will probably be found nearly such as is represented in the calculations subjoined in the note, deduced from data, which have been obligingly furnished by an acute and experienced member of the Navy Board."

By the above calculation, it will appear that there is an excess of weight at both extremities, and that in the adjoining sections the pressure greatly preponderates. Consequently, these forces are opposed to each other, and in a direction very prejudicial to the ship; as it is not only the evil of weight preponderating at the extremities, the power of which will be increased by the action of the ship, but from the formation of the body at those parts they can afford but a feeble resistance to so great a force.

Perhaps the following figures will best explain where the weights and pressure now preponderate, what particular weights may be supposed to cause an excess, and such as may be most conveniently and advantageously transferred.

* "In a modern 74-gun ship, fitted for sea, the length being 176 feet, the breadth 47, the the forces are thus distributed.

[merged small][merged small][merged small][merged small][ocr errors][merged small]
« AnteriorContinua »