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which, in the state of nature, constitutes its epidermis, and it has no power of forming this sub stance, which there is good reason for supposing elementary. Mr. Davy gave an account of the experiments which shew that carbonic acid is absorbed and decomposed by plants in the solar light, and oxygen evolved. He seemed inclined to doubt whether they ever evolved carbonic acid in a state of health; and he mentioned some facts, which seemed to shew, that the carbonic acid which usually appears when plants are confined in darkness in close vessels, is really owing to the decay of some of their dead parts. The epidermis, the heartwood, or a single yellow spot in the leaf, would be fully adequate to such an effect.
ascent of the stalk, and descent of the radicle, which seem to shew that gravitation is the principal cause of both these effects. The chief diseases of the more perfect plants, he stated, are produced either by parasitical vegetables, or by insects. Wet seasons conduce most to the propagation of mildew or blight; and dry weather to the increase of the turnip fly, and other analogous destructive insect tribes.
The eighth, and concluding Lecture of the course was upon the mode of the dissemination of seeds, and upon the progress of vegetation, in a state of nature. Rocks, according to Mr. Davy, by their decomposition, form a soil; different species in very different periods; lichens and mósses are their first productions, and lastly a mould is formed capable of supporting grasses. Peat, he considered, as chiefly arising from the destruction of forests, exposed by the early cultivators of dif
The seventh Lecture was principally devoted to the consideration of the causes of germination and the circumstances that affect the healths of plants. Mr. Davy stated that seeds were inca-ferent countries, by thinning their outskirts.pable of germinating, unless supplied with heat, moisture, and air, and that oxygen is always absorbed in this process, and carbonic acid evolved. He mentioned Mr. Knight's experiments on the
Mr. Davy made some general observations on the nature of different soils, and recommended new inquiries on this subject as peculiarly important to the agriculture of the country.
THE famous Agrippa, in his treatise on the most learned French authors, Du Chesne, La vanity of the sciences, has collected many instan- Laboreur, Chifflet, Fauchet, and Father Menesces of these marks of distinction; the Romans trier; according to Cambden, the use of family bore the eagle; the Phrygians, a hog; the Thra arms began among the English, as well as French, cians, a skeleton, or the figure of death; the just after the crusades, yet, if we may rely on the Goths, a bear; the Alans, that invaded Spain, a learned Sir Henry Spelman, hereditary bearings cat; the old Franks, a Lion; and the antient were not generally established until the time of Saxons, a horse, which is still borne in the arms Henry III. of England; for the last Earls of of his Britannick Majesty; but these marks were Chester, the two Quincies, Earls of Winchespromiscuously taken for hieroglyphics, symbols, ter, and the two Lacies, Earls of Lincoln, still emblems, and personal devices, like the salaman- varied the son from the father. As for the Scots der of Francis I. of France, and were not arms and Welch, they pretend to excel the English properly so called. Thus Pasquier tells us in his and French in ancient descents, and regular Recherches de la France, or inquiries into the armory; but according to Father Menestrier, antiquities of France, lib. ii. p. 84. that before whose authority is esteemed of great weight in this Marius, the eagle was not the constant ensign of matter, Henry the Falconer, who was raised to the Roman Generals, who, in their standards bore the Imperial throne of the West in 920, by regu sometimes a wolf, sometimes a leopard, or anlating the tournaments in Germany, gave occa→ eagle, according to the fancy of the chief comman- || sion to the establishment of family arms, or marks der. The like variety is observed in the arms of of honour, which undeniably are more antient, the King of France, and Great Britain, as we shall and better observed among the Germans, than in mention hereafter, but the most learned authors any other nation. Moreover, according to Father agree, that the hereditary arms of families, as well Menestrier's opinion, with tournaments first came as their double names, or surnames, began no up coats of arms, which were a sort of livery, made sooner than the crusades, that is about the begin-up of several lists, fillets, or narrow pieces of stuff ning of the tenth century; and their opinions who of divers colours, from whence came the fefs, the trace them up higher is confuted by the best and "bend, the pale, the cheveron, the lozenge, &c. No. XV. Vol. II.
which were the original of family arms; for they who never had been at tournaments, had not such marks of distinction, although they were gentlemen. They who inlisted themselves in the crusades, for the conquest of the Holy Land, took up also armorial ensigns, particularly crosses of divers colours, for distinction sake; before that time, that is, before the tenth or eleventh century, nothing is to be seen on the more antient tombs but crosses and Gothic inscriptions, with the effigies of the person; the tomb-stone of Pope Clement the Fourth, who died in the year 1628, is the first on which a coat of arms is found; nor are arms to be seen on seals or coins older than the tenth or eleventh century. The first French coin with arms is a golden denier of King Philip de Valois, on which he is represented holding with his left hand a scutcheon semée of fleurs de lis. This piece of gold, coined in 1966, was called in French ecce, by reason of its bearing the escutcheon of the arms of France. There are, indeed, more antient figures to be seen, either in standards, or medals, but neither Princes nor cities, made use of them, as formal or regular bearings; and no author of note mentions the heraldic science above those ages: to all this may be added, that it is very probable, this art, like most human inventions, was insensibly introduced and established, and that having remained in a rude and unsettled state for many ages, it was at last perfected and fixed, by the crusades and tourna
As to the name of blazonry, authors differ no less about it than about the origin of the art itself; some, by a metathesis, derive it from the Hebrew sobal, which in Latin signifies, tulit, portavit, "he has borne;" others with greater consonance, but as little reason, deduce it from the Greek BLASTEIN, which in Aristosle signifies in Latin, distorquere, and in English, to wrest, distort, to set awry; and, taken more extensively, to extravagate, or rave; because, say they, in antient times, they who were not initiated into heraldic mysteries, looked upon most of the
figures and ornaments of the shields of Cavaliers, as extravagancies. Menage, whose etymologies are forcibly dragged in, fetches the word, blazon, from the Latin latio, a bearing, by putting before it a b, because blazon denotes, "what's borne on a shield." Borell, hits it a little better, in deriving it from two Latin words, viz. laus, praise, and sonare, to resound; and, by putting a b before the entire word, out of that odd composition he forms the barbarous verb, blausonare, from whence, at last, he draws the substantive, blauson, or blazon. Others, with far greater plausibility, deduce it from the English, to blaze, which in a proper sense, signifies to flash, to burn with, or cast a great flame, and in a figurative, and active signification, to set forth, to publish, to proclaim; but the most general, as well as the most rational opinion is, that both the word blazon, and the English, to blaze, comes from the German blafen, that is, to sound a horn, or a trumpet; because the knights and nobles who came to enter the lists at tournaments, caused those instruments to be sounded, to proclaim their arrival; whereupon the Heralds sounded also their trumpets, and then blazoned the coats of arms of the tilters; that is, displayed and described them aloud, and sometimes expatiated on the praises of the bearers. Hence, probably, it is that the word, to blazon, formerly signified, to display or set forth a man's ill or good qualities, or to give a character of him but now use restrains it to an ill sense; for by blazoning a man, we only mean to expose him, to display him in his proper colours, to speak ill of him, &c.; but here it is to be observed, that some pretend, that in the last signification, the verb, to blazon, is more antient than the heraldic rules, or blazonry itself; and, to support their opinion, they alledge, that when the knights of the shield received their order, they were enjoined, not to suffer ladies to be blazoned; that is, slandered or exposed, in their hearing. [To be continued.]
ON PNEUMATICS. [Concluded from Page 97]
ON THE RESISTANCE OF AIR AS A MEDIUM.
MEDIUM denotes that space, or region, through which a body passes in its motion towards any point. The air is a resisting medium, and the resistance it opposes to a body moving is proportioned to the surface which that body offers.
If in a room a feather and a guinea be dropped from the hand at the same instant, the guinea will reach the floor immediately, while the feather descends gently, and with an indirect motion, on account of the resistance it meets with from the air contained in the room. Were they dropped from the hand in vacuo, the time
of descent would be the same in both; for when
The two mills A and B have each an equal number of sails, whose weight, length, and breadth are in each precisely equal. The one has its sails fixed edge ways, so as to cut the air with only a thin edge: the other offers the whole breadth of its vanes to the air. On giving the sails of both an equal impulse, they will begin to turn round with equal velocity; but the one which presents the whole surface of its vanes soon begins to move slower, and at length stops, while the other still continues in motion. The reason is obvious; this last, offering no greater surface to the air than the edge of the sails, finds less resistance, and therefore obeys the impulse it had received for a longer time.
An arrow which offers its point to the air flies a considerable way, but an arrow whose side is opposed to the current of air falls immediately.
If a ball, and a quantity of shot equal to it in weight, be discharged from two guns at the same instant, and with the same velocity, the former will be sent to a much greater distance than the latter, for the sum of the surfaces of the shot greatly exceed the surface of the ball.
Winged animals are incapable of flight in vacuo; for as they make use of the resistance of the air to facilitate their motions (in the same way as fishes make use of the water by striking it with their tails), when no such resistance offers, their wings are useless. If a butterfly be suspended from the middle of a receiver by a thread fastened to its horns, it will fly about with ap, parent ease so long as the receiver remains filled with air, but no sooner is the air extracted from it than the butterfly hangs perpendicularly, incapable of raising itself by any efforts that it makes.
Smoke, being a lighter fluid than air, generally ascends; but in moist and hazy weather it is seen to fall, for then the air possesses less density, therefore can make less resistance to the tendency
| which smoke, in common with every other body, has to gravitate towards the earth.
When a sonorous body is struck, a tremulous motion is communicated to all its parts, and by their vibration to the air, which carries the impression forward to the ear. Hence three things are necessary to the production of sound,-a sonorous body to give the impression, a medium to convey it, and an ear to receive it.
That a vibratory motion is produced in the parts of a sonorous body when it is struck, may be found by laying the hand on a bell, or a pair of tongs, when either have received a stroke. In both cases a tremulous motion will be felt in the parts beneath the hand. As long as this vibration continues, a correspondent motion is produced in the air, which motion is well illustrated by the circles caused in water on throwing any substance into it; as these circles extend themselves in every direction, so do the parts of the air that surround a sonorous body when this last receives a stroke; each part communicates the motion impressed upon it to the portion of air next it, and thus are generated a succession of waves that float the sound to the ear, in whatever direction it may be placed.
Sound, whether it be loud or feeble, moves always with the same velocity by night or by day, in hot weather or cold, except that its velocity is a little impeded or accelerated by strong currents of air; but the distance to which sound is carried depends on the force of the impression made on the air. Sound is not instantaneous but progressive; it travels at the rate of thirteen miles in a minute, or 1142 feet in a second. Every body knows that the flash of a gun is seen before the report is heard, and that lightning sometimes precedes the thunder several seconds; the flash and the report are nevertheless generated at the same instant; but the former reaches us with the velocity with which light travels, that is, at the rate of 200,000 miles in a second of time, whereas the latter travels at the rate of only 1142 feet in the same period. Availing ourselves of this knowledge, we may at any time ascertain our distance from the seat of a storm, by counting on a stop-watch the number of seconds that elapse between the flash of lightning and the thunder. Suppose, for example, the latter be heard five seconds after seeing the former, then the report has travelled over five times 1142 feet, or something more than a mile. As a stop-watch is not always to be had, the calculation may be made by means of the pulse at the wrist; in every healthy person this commonly beats about seventy-five times in a minute,
and in one beat of the pulse of such a person sound passes over the sixth part of a mile, consequently in six pulsations it will pass over a mile.
Air, though the common conductor of sound, is not the only one; water conveys it to the ear much more strongly. The unassisted human voice has been heard over water to the distance of ten or twelve miles; a person has been heard to read equally as distinctly at the distance of 140 feet on water as he could be heard on land at the distance of 76 feet. The Romans were so well acquainted with the property of water to encrease the force of sound, that they had a canal of water carried under one of their theatres which was too large to admit of the voices of the performers being heard in the remoter parts of the building. It appears from various experiments that, in general, the denser the medium is, the more intense is the sound; flannel, however, is an excellent conductor of sound; put a narrow slip of it round the middle of a poker, then roll the ends of the flannel round the first fingers of each hand, and put these fingers in the ears; strike the poker against the fender, or any hard substance, and a sound will be produced which equals in loudness that generated by the largest church bells; at the same time the poker, if the hand be applied to it, will be found to make a number of sensible vibrations.
That wood is a conductor of sound the variety of musical instruments made of it fully evinces; the slightest scratch with a pin, at one end of a long piece of timber, will be distinctly heard by an ear applied to the other.
In music, the tone of a sound depends on the time that a string vibrates; some strings have long vibrations, and produce deep or grave tones; hence the different notes which a violin or harp are capable of producing. If a long musical string be divided into two equal parts by a bridge, each half will produce a note eight times higher than that which issued from the string before; hence it appears that the vibrations which are inost rapid produce the sharpest sounds.
A sort of sympathy exists between bodies in a state to produce accordant sounds. If strings in unison are placed near each other, both will sound when one is struck; even if the distance between them be two or three feet the same thing will occur; when the strings are not in unison, no such effect takes place. A wet finger pressed round the edge of a drinking-glass, will produce its key; if the glass be struck so as to produce its pitch, and an unison to that pitch be strongly excited on a violincello, the glass will be set in motion, and if near the edge of the sable, will be liable to be shaken off.
OF THE ECHO.
Echoes are caused by the reflection of those undulations in the air by which sounds are propagated. When a pebble is thrown into a pool, the water which receives its impression recedes on all sides towards the margin, having reached which it is driven back. The same undulations are produced in the air by a stroke on a sonorous body, or by the voice, and when in their retrocession from the point of impulse the undulations encounter a rock, a house, or any other similar surface, they are reflected or driven back, which causes an echo; to hear this echo it is necessary, however, that the ear be in the line of reflection.
Suppose C to be the generating point of sound, or the point where motion is given to the air by the voice, or a stroke, or any sonorous body, the air will immediately recede in a straight line towards the rock P, which will reflect it back again by the same line, and produce, to a person standing at C, an echo of the original sound. But if the generating point of sound were at A, the undulations would reach the rock by the oblique line A d, and be reflected by the oblique line d E, so that no echo would be heard by an ear at A or at C, while a person standing at E would hear one because he is in the line of reflection.
It sometimes happens that an echo is heard by a person whom the direct sound did not reach. If between A and E there were a hill, this would be the case with a person standing at E when sound was generated at A; for though the hill would prevent the undulations in the air reaching him in their retrocession from A, and consequently his hearing the direct sound, yet when reflected by the rock they would be carried to his ear, and he would hear the echo. An echo, however, supposes that there is a distance of seventy or eighty feet between the generating point of sound and the reflecting surface; if the space be less no echo can be produced.
There are echoes which reflect the sound sereral times; this happens when there are a number of walls, rocks, &c. whence the sound is reflected from one to another; but for a person to
hear these repeated echoes, he must be so situated as to intercept the undulations each time they are reflected. At Roseneath, near Glasgow, there is an echo that repeats three times a tune played with a trumpet. Near Rome there was one that repeated five times any thing which was said. At Brussels there is an echo which answers fifteen times. Between Coblentz and Bingen there is one which differs from most others; in common echoes the repetition is not heard till some time after the words have been uttered, in this the person who speaks is scarcely heard, but the repetition is clear, and surprisingly varied; the echo in some cases appears to be approaching, in others receding, and sometimes it is heard distinctly, at others scarcely at all; one person hears but one voice, while another hears several. At a place near Milan, in Italy, the sound of a pistol is returned thirty-six times.
The phenomenon of a whisper made against the wall on one side of the gallery at St. Paul's cathedral, is thus explained:-the wall being extremely smooth is an excellent conductor of sound; the undulations made in the air by the voice are reflected both ways by the wall, and meet at the opposite side; to the hearer, therefore, the effect is the same as if his ear was close to the mouth of the speaker,
OF THE BAROMETER AND THERMOMETER.
The barometer is an instrument in such general use that a description of it is unnecessary; the upper part of the tube is a vacuum, thus formed: the tube, which is closed at one end prior to its being fixed to the frame, is com. pletely filled with mercury or quicksilver; every particle of air is, of course, expelled from it; a finger being applied to the open end of the tube, to prevent the mercury from running out, it is then inverted and plunged into a cup containing mercury; that in the tube then subsides three or four inches, leaving a space above it which is a perfect vacuum. The tube, and the cup in which it is immersed, are then attached to the graduated frame of the barometer; and as there is no pressure of air on the upper surface of the column of mercury within the tube, the pressure of the atmosphere on the surface of that contained in the cup forms a counterpoise to the included column, and keeps it from running out. When, however, the atmosphere is less dense than usual, the column sinks a little, the air having somewhat less weight; when the atmosphere is more dense, the pressure upon the mercury in the cup being greater, that in the tube rises. These variations are included within about three inches; in our climate the least ele
vation of the mercury is twenty-eight inches, and its greatest thirty-one.
Since the suspension of the mercury in the barometer is occasioned by the pressure of the atmosphere, and since the pressure decreases from the earth upwards, it follows that the column of mercury ought to be shorter at the top of a high mountain than at its base. This is actually the case; in every hundred feet of perpendicular ascent the mercury sinks about the tenth of an inch, consequently at the height of a thousand feet it will descend a whole inch; and thus, by taking a barometer to the top of a mountain, or any other considerable eminence, we may ascertain its perpendicular elevation.
The thermometer is used for marking with precision the changes which take place in the temperature of the atmosphere. Like the baro. meter, it consists of a tube closed at the top, or, in technical language, hermetically sealed, and fixed to a graduated frame. It is constructed on this principle, that fluids of every description, and mercury in particular, expand by heat, and are contracted by cold. If a thermometer be plunged into boiling water it will rise to 212°; this is called the boiling point. If it be plunged into melting ice, it will fall to 32°; this is termed the freezing point. The utmost extent of the mercurial thermometer, both ways, are the points at which quicksilver boils and freezes; it boils at a degree of heat equal to 600°, and freezes when it is reduced as low as 39° or 40° below 0, consequently the whole extent of the mercurial thermometer is 640 degrees.
Of the HYGROMETER, AND THE RAIN-GAUGE.
The hygrometer is used for ascertaining the different degrees of humidity in the atmosphere. The most simple instrument for this purpose, and one sufficiently accurate for common observation, may be made by suspending from a wall a weight fastened to a piece of twisted catgut; in damp weather the catgut will contract, in dry weather it will extend, and the difference, or variation, may be shewn by fixing a scale of equal parts on the wall near the weight, and a small index to the calgut.
A piece of spunge makes a very good thermometer, particularly when, after having been freed from all impurities by being washed, it is dipped into water in which sal ammoniac, or any other salt, has been dissolved; for, when the spunge is dried, the saline particles imbibe the moisture, and the sponge will every day vary in weight according to the different degrees of moisture in the atmosphere.
The rain gauge shews the height to which rain