Imágenes de páginas
PDF
EPUB

for the accomplishment of her purposes. The act of feeding is continually effected in this manner. The operation of sucking is too familiar to require comment. It may not, perhaps, be so generally known that it is by the very same process that bees reach the fine dust and juices of hollow flowers, like the honeysuckle, and some species of foxglove, which are too narrow to admit them. They fill up

the mouth of the flower with their bodies, and suck out the air, or at least a large portion of it, by which the soft sides of the flower are made to collapse, and the juice and dust are squeezed towards the insect, as completely as if the hand had pressed it externally.

NOTE 3. p. 41.

Those who are not acquainted with the operations by which the mind is enabled to arrive at truth, are too apt to attribute to accident that which is the result of great intellectual labour and acuteness. Observation, analogy, and experiment are the three great stepping-stones by which the philosopher is enabled to ascend from darkness to light: it is true that his foot may accidentally be placed upon the first, but his own efforts are required to complete the ascent. To the mass of mankind the preliminary step is obvious, and they, at once, conclude that the succeeding ones are equally easy and simple. In this view of the subject, it was by accident that Sir Isaac Newton discovered the laws of gravitation, for his mind was directed to the investigation by the accidental fall of an apple from its tree; it was by accident that Galileo discovered the isochronous movement of the pendulum, for it was suggested by the vibration of a chandelier: but how many persons might have witnessed the fall of an apple, or the vibration of a chandelier, without arriving at similar truths ? It has been said that we are indebted for the important invention in the steam-engine, termed hand geer, by which its valves or cocks are worked by the machine itself, to an idle boy of the name of Humphrey Potter, who, being employed to stop and open a valve, saw that he could save himself the trouble of attending and watching it, by fixing a plug upon a part of the machine which came to the place at the proper times, in consequence of the general movement. If this anecdote be true, what does it prove? That Humphrey Potter might be very idle, but that he was, at the same time, very ingenious. It was a contrivance, not the result of accident, but of acute observation and successful experiment. We are endeavouring to combat a popular but mischievous error; and we are happy at finding the same feeling expressed in a work which, from its extensive circulation, must prove highly useful in correcting it.

* Very few discoveries,” says the author, “ have been made by chance and by ignorant persons; much fewer than is generally supposed. They are generally made by persons of competent knowledge, and who are in search of them. The improvement of the steam-engine by Watt, resulted from the most learned investigation of mathematical, mechanical, and chemical truths. Arkwright devoted many years, five at least, to his invention of spinningjennies. The new process of refining sugar, by which more money has been made in a shorter time, and with less risk and trouble, than was perhaps ever gained by an invention, was discovered by Mr. Howard, a most accomplished chemist, and it was the fruit of a long course of experiments, in the progress of which, known philosophical principles were constantly applied, and one or two new principles ascertained.”- Library of Useful Knowledge.

NOTE 4. p. 58.

If a soap-bubble be blown up, and set under a glass, that the motion of the air may not affect it, as the water glides down the sides and the top grows thinner, several colours will successively appear at the top, and spread themselves from thence in rings down the sides of the bubble, till they vanish in the same order in which they appeared; at last a black spot appears at the top, and spreads till the bubble bursts. Hence it follows that the colours of a body, depend in some degree upon the thickness and density of the particles that compose them; and that if the density be changed the colour will likewise be changed. That the production of colours depends upon the nature of the surfaces upon which light falls, is beautifully exemplified by the iridescence of mother of pearl; and which has been satisfactorily shown to depend upon a singular peculiarity in the structure of that substance. On its surface, which to the unassisted eye, and even to the touch, appears to be finely polished, there are innumerable little lines, or grooves, in some places as many as two or three thousand in the space of an inch, which, lying parallel, regularly follow each other in all their windings; by the edges of which the rays of light are reflected, and the continual change of colour arises from their continual bendings. Whatever doubts might have existed upon the subject, some late experiments of Dr. Brewster have dissipated them, by showing that the colours which play so beautifully on the surface of mother of pearl, may be communicated by pressure to sealing-wax and several other substances. The discovery of this fact was in some measure accidental; he had stuck a piece of mother of pearl on a cement made of rosin and bees-wax, and on separating this cement he found that it had acquired the property of exhibiting colours. Several persons who witnessed the effect, concluded that it arose from the presence of a thin film of the mother of pearl, which might have scaled off and adhered to the wax: but such an explanation was at once refuted, by plunging the wax in acid, which must have dissolved the mother of pearl, had any been present; but the acid had no effect, and the colours of the impression remained unimpaired. It is clear, then, that it is the grooves, as Dr. Brewster conjectured, which occasion the iridescence in the mother of pearl, as well as in the waxen impression. In consequence of this curious discovery, Mr. Barton succeeded in producing the same appearance on glass, and on different metals, by simply cutting grooved lines on their surface. These lines are so fine that, without a microscope, they are scarcely visible, and the glass and the metal appear to retain their polish; yet they and the colours also may

be comm icated by an impression, like those from the mother of pear), to the wax.

NOTE 6, p. 90.

The following are a few of those plants which indicate changes in the weather :

Chickweed is an excellent barometer. When the flower expands fully, we are not to expect rain for several hours; should it continue in that state, no rain will disturb the summer's day. When it half conceals its miniature flower, the day is generally showery; but, if it entirely shuts up, or veils the white flower with its green mantle, let the traveller put on his great coat. The different species of trefoil always contract their leaves at the approach of a storm ; so certainly does this take place, that these plants have acquired the name of the husbandman's barometer.

The tulip and several of the compound yellow flowers also close before rain. There is, besides, a species of wood-sorrel, which doubles its leaves before storms and tempests. The bauhinia, or mountain ebony, cassia, and sensitive plants, observe the same habit.

NOTE 7. p. 92.

The popular adage of Forty days' rain after St. Swithin, is a tradition which seems to have derived its origin from the following circumstance. Swithin, or Swithum, bishop of Winchester, who died in 868, desired that he might be buried in the open churcb-yard, and not in the chancel of the minster, as was usual with other bishops; and his request was complied with : but the monks, on his being

« AnteriorContinuar »