Professor Lindley, on the durability of plants immersed in water. Having immersed in a tank of fresh water, during more than two years, one hundred and seventy-seven species of plants, including representatives of all those which are either constantly present in the coal-formations, or universally absent, he found,

“1. That the leaves and bark of most dicotyledonous plants are wholly decomposed in two years, and that of those which do resist it the greater part are Coniferæ and Cycadeæ.

“ 2. That Monocotyledons are more capable of resisting the action of water, particularly palms and scitamineous plants; but that grasses and sedges perish.

“ 3. That fungi, mosses, and all the lowest forms of vegetation, disappear.

6 4. That ferns have a great power of resisting water, if gathered in a green state, not one of those submitted to the experiment having disappeared; but that their fructification perished.

“ Although the results of this experiment, in some degree, invalidate the certainty of our knowledge of the entire Flora of each of the consecutive periods of geological history, it does not affect our information as to the number of the enduring plants which have contributed to make up the coal-formation ; nor as to the varying proportions, and changes in the species of ferns and other plants, in the successive systems of vegetation that have clothed our globe.

“ It may be further noticed, that as both trunks and leaves of Angiospermous dicolyledonous plants have been abundantly preserved in the tertiary formations, there appears to be no reason why, if plants of this tribe had existed during the secondary and transition periods, they should not occasionally have escaped destruction in the sedimentary deposits of these earlier epochs.”—P. 48).

Dr. Buckland has not dwelt sufficiently on the various causes which influence the preservation in strata of parts of organic beings; nor on the circumstances that cause a far greater Lumerical proportion of the remains of certain classes and orders to be found than of others. Without this commentary, the nature and value of the evidence derived from this source cannot be properly appreciated; unless thus forewarned, the first hasty conclusion which would be drawn, is, that the proportion between the numbers of species of which the remains are preserved, was the same as that of the species existing at the period.

But when it is shown that marine animals, generally, are placed in circumstances most favourable to the preservation of their solid parts, and conchiferous and molluscous more so than fish, and these again more than amphibious reptiles,—and that it must require a rare combination of circumstances to allow of the skeletons of birds, or the exuviæ of insects, being preserved from that decomposition to which the most solid parts of all organic beings are subject from exposure to atmospheric influence; the student is made aware of the caution necessary to be used in drawing inferences as to the state of living creation at any former period, from the fossil remains of that time. Perhaps in the strong cases we have cited in illustration, his unaided judgment would not far mislead him; and he would infer from analogy, the existence of a proportional number of insects at all periods of the Creation, to serve as the intermediate agents in the ceaseless routine of conversion of animal into vegetable bodies, destined again to furnish nutriment to the former; but it is necessary he should constantly have these general principles present to his mind, to apply before he draws any conclusions on the subject of the numerical proportion of living beings of the most nearly related orders, since, even among these, habits and modes of life may concur to favour the preservation of the remains of certain genera, while those of others may strongly militate against the preservation of any records of them.

We are here obliged to close our remarks, and we are less reluctant to do so, knowing how much has appeared elsewhere in our own as well as in the pages of cotemporary works; the volumes are, indeed, replete with the most interesting and valuable information on the subject of fossil remains; and our regret that Dr. Buckland did not make the plan of his work more comprehensive, shows how highly we estimate both the extent of his knowledge and his powers of instruction.

We will not conclude our notice without expressing admiration at the taste and judgment shown in the selection and execution of the plates. The wood-cuts, designed and engraved by Mr. Fisher, place him in the first rank of his invaluable profession, and the exquisite skill and talent of Mr. J. D. C. Sowerby as an artist are well known. Dr. Buckland will not take it as a bad compliment, if we say that his second volume alone would entitle him to the gratitude of every naturalist and geologist.

THE PHILOSOPHY OF THE HOUR-GLASS.

In olden time, long ere the art of clock-making was discovered, our ancestors marked the fleeting hours by the flowing of sand in a glass. This contrivance was called the hour-glass, and it is still very generally to be found upon the table of the public lecturer, or the private teacher, in the laboratory of the philosopher, or in the cottage of the peasant. It is a far more accurate measurer of time than is usually imagined, and, therefore, perhaps a short account of the theory of its action may be acceptable to the readers of the Magazine of Practical Science.

The investigation was undertaken a few years since by M. H. Bournand: his experiments are exceedingly curious, and merit to be more generally known. A few only of the most remarkable, and easy of performance, are detailed in the following notice.

The first remarkable fact regarding the hour-glass is, that the flow of its enclosed sand is perfectly equable, whatever may be the quantity contained in the glass, at any period of its flowing: or, in other words, that it runs no faster when the upper cone is quite full, than when it is nearly empty. This is contrary to what we might expect, for it would be natural enough to conclude, that when full of sand, the lowest particles would sustain a greater pressure from the incumbent mass, and, therefore, be more swiftly urged through the aperture, than when only a quarter full, and near the close of the hour.

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The fact that the flow is equable, at any period, may be proved by a very simple experiment.

Provide a quantity of what is called silver-sand, (well known for domestic use,) dry it upon a hot stove-plate, or in an iron ladle over the fire; then sift it through a tolerably fine sieve, carefully removing all lumps of clay or stone, which are frequently found in it. Next, take a tube of any material, length, or diameter, closed at one extremity, and in the bottom make a small aperture, say the eighth of an inch, place the finger over this, or stop it lightly with a small plug, and then fill up the tube with the sifted sand.

Hold the tube steadily, or affix it to a wall, or a frame, at any convenient height from a table, and then removing the finger, or plug, permit the sand to flow in any measure, for any given time,-supposing into a common graduated glass measure, for a quarter of a minute. A certain quantity is thus obtained, which must be noted. Now let the tube be only a half or a quarter full of sand, and begin to measure again, for a like time, the same quantity of sand will flow; and even if by means of

ler or plug, the sand in the tube is violently pressed upon, the flow of the sand from the aperture will not, in the least degree, be accelerated, provided the tube is kept steady, and the experimental comparisons all accurately and fairly made. Now all this admits of a simple and satisfactory explanation. Sand, if allowed to fall quietly upon any surface, will form itself in a conical heap, having an angle of about 30°; this is seen in the lower cone of the hour-glass, or can be shown by letting the sand fall from the aperture of the tube just mentioned. Whenever a load of dry sand is thrown from a cart or barrow, or sifted through a screen, by the builder in making mortar, it forms a like conical heap, having an angle of 30° or 35o. Now, then, observe the application of this fact of every day occurrence; it will show how intimately “ things familiar” are connected with practical science.

As sand thus falls at a determinate angle, it is easy to imagine that when poured into the tube, it must fill it with a succession of conical heaps, and that all the weight which the bottom of the tube sustains, is only that of the heap which first falls upon it, and that the succeeding heaps are thus prevented from exerting any perpendicular pressure upon the bottom, but that they only exert a lateral pressure against the walls of the tube.

When pressure is applied to the top of the sand, it is only transmitted laterally, and that to a very little extent; consequently the lowest heap of sand enjoys its flow uninfluenced by the strata or pressure above it. This is the reason why the hour-glass flows with such regularity; and that any given base sustains no weight of sand but that of the first heap which falls upon it, and is in immediate contact with it, may be proved by taking a tube, about an inch diameter, open at both ends, wetting with the lips the edges of a small piece of tissue-paper, and applying this to one end of the tube, so as to form a bottom held on by a very slight adhesion. Fill this tube carefully with any weight of sand, and the paper will not be forced away, not even if with a round ruler or rod great pressure is applied to the top surface of the sand. All the weight of sand that the tissue-paper supports is the little heap which first falls upon

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If the experiment is made upon a larger scale, with tubes three or four inches in diameter, and five or six feet long, it is better to paste the paper round the bottom, because the first heap in such case would be of considerable weight; but if the paper is strong enough to resist it, forty or fifty pounds of sand may be poured into the tube, and all lifted from the ground together, without the slightest fear of the paper being forced away.

The experiment admits of another modification. Take the tube open at both ends, and place one of them in contact with the bottom of a small cup floating upon water, then fill up the tube with sand; none of it will run out into the cup: and that there is no perpendicular pressure is evident from the cup still continuing to float; it sustains no weight save that of the first heap,—the hand of the operator sustains the weight of all the rest of the sand. Draw away the tube, the sand then rushes into the cup in obedience to the law of gravitation, and its weight causes the cup

to sink.

From such experiments it may be concluded that it must be extremely difficult to Thrust sand out of a tube, by means of a fitting plug or piston; and this upon trial is found to be the case. Fit a piston to a tube, (exactly like a school-boy's pop-gun,) pour some sand in, and try with the utmost strength of arm to push out the sand. It will be found impossible to effect this; rather than the sand should be propelled, the tube will burst laterally.

Directions are often given by naturalists for shooting birds with a a charge of fine sand, so that their plumage may not suffer the damage which is usually occasioned by an ordinary charge of small shot. This proceeding ought to be made with considerable caution, for it must be evident, from the last experiment, that a charge of sand would resist the expansive force of gunpowder, and violently strain the barrel, perhaps burst it; to say nothing of goring, and spoiling its polished interior by the rapid friction of the sand; and, supposing it in very small quantity, and successfully propelled, it is certainly rather a hazardous experiment. To

prove how a small column of sand will resist the expansive force of a large charge of gunpowder, it will be sufficient to instance the method adopted by engineers for blasting rocks.

A hole is drilled in the rock, of the requisite depth, at the bottom of which the charge of powder is placed; a long match, or reed, filled with powder, is then put down, and around this sand is merely poured in, so as to fill the hole; a train is laid and fired, and presently the explosion takes place, rending away the mass of rock. The loose column of sand is not blown out of the hole leaving the rock unshaken, but it keeps its place, until it compels the solid rock to yield unto its singular power.

The discoverer of these facts, relating to the flow of sand in the hourglass, makes the following observations.

“There is, perhaps, no other natural force on the earth which produces by itself a perfectly uniform movement, and which is not altered either by gravitation, or the friction, or resistance of the air; for the height has no influence, friction in place of being an obstacle is the regulating cause, and the resistance of the air within the column must be so feeble as to be altogether insensible as a disturbing force.”

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A POPULAR COURSE OF ASTRONOMY.

No. VI.

DAY AND NIGHT.—THE SEASONS.

the sun.

THERE have now been placed before the reader those first truths of astronomy on which the whole fabric of the science may be considered to rest. The infinite distance of the region of the fixed stars, the entire isolation of the earth in space, its spherical form and huge dimensions, its daily revolution round one of its own diameters, and its annual revolution round

In this chapter, some of those great phenomena of the visible world which result from these will be brought under his consideration. First among these are the alternations of day, of night, and the changes of the seasons. The sun is the source of light and heat ; these are facts of which the experience of every day of our lives constitutes the demonstration; they are so plain and palpable that no one was probably ever found to deny them. It is a matter also of daily experience, that a certain class of bodies called opaque, to which class belong by far the greatest number of the bodies around us, have the power of obstructing the light, and also, in a great measure, the heat of the sun; so that whilst the light and heat fall and exert their full influence on one side of them, the opposite is wholly deprived of that influence; the one side is then said to be enlightened and heated, and the other, where there is an entire absence of light and heat, to be in a state of darkness and cold. Now let us suppose a body thus opaque to be turned round, so that what was before the part turned towards the sun, may now be that from it; that part will be found to have retained none of the light which it received in its first position, so as to be now wholly and absolutely in a state of darkness; but it will, on the contrary, have retained a larger portion of its heat, so as not to be wholly and absolutely in a state of cold. Let us suppose the body a sphere; then will the enlightened portion of it be a hemisphere, that is, one half of the whole, and the division of the light and the dark part of it will be a great circle, A CB D, of the sphere. If the sphere be turned round one

the diameters, A B, of this circle, the positions of its light and dark parts will eventually and gradually be interchanged,—the c whole of what was dark before will now be light, and the whole of what was light before, will be dark ; and if the revolution of the sphere be continued uniformly, that is, always with the same velocity, then each point in it will continue as long on the light side as on the dark side,—as long on the side on which it receives heat, as on that on which it does not receive it. The quantity of light and heat which any point receives during the time of its revolution through the enlightened hemisphere is not, however, the same in all the positions which it may be made

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