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hours, minutes, &c., it is before or after the meridian transit that the rocket will be thrown up, we are manifestly just as well able to tell what was the precise moment of transit as though the rocket went up when it actually took place.
Well now, instead of the throwing up a rocket, let us suppose that the astronomer at Greenwich could put a screen over the moon, or over one of the satellites of Jupiter, precisely at a number of hours before or after the transit of the sun at Greenwich, which number of hours was known and agreed upon beforehand. This phenomenon, wherever it was visible, would answer the purpose of the rocket, and enable us to tell the time of the transit of the meridian of Greenwich; comparing which with our own, we should know the longitude.
Now this is what actually occurs, except that it is not the hand of the Astronomer, but that of God, which, at appointed seasons, brings darkness
upon the face of the moon, and causes night after night one or other of the satellites of the planet Jupiter to plunge into his shadow. The precise number of hours, minutes, and even seconds, before or after the transit of the sun at Greenwich, when these phenomena occur, are calculated and registered in the Nautical Almanack; and any one observing them at ever so distant a place, can tell thus the time of the sun's transit at Greenwich, and observing his own, he can thus find his longitude.
The eclipses of the sun or moon occur but rarely, so that an opportunity of finding the longitude by them, although certainly the best, is very seldom presented to us. The eclipses of Jupiter's satellites occur almost nightly, and these answer every purpose of finding the longitude on land. But at sea this method fails, for it is impossible to hold a telescope of the required length sufficiently steady on ship-board to see the satellites of Jupiter.
In the failure of these methods, another of great ingenuity has been contrived-a method which, in its practical application, simplified as it is by the use of tables, presents little or no difficulty, but which, in the researches on which those tables are based, constitutes one of the greatest triumphs that the human intellect has in our times achieved.
The moon does not rest among the fixed stars, but moves along that band of the heavens which is called the zodiac; and it moves with a comparatively rapid motion, describing the complete circuit of the heavens in 27d 7h 43'4", or moving at the mean rate of 13° 10 35" a day, and 32' 56.46" per hour. Thus, then, the moon is in no two successive hours at the same distance from any one of the fixed stars, nor, indeed, in any two successive minutes. Suppose, now, that the time after or before the sun's transit at Greenwich, when the moon would be at a given angular distance from a certain fixed star, were calculated, and inserted in the Nautical Almanack, and that an observer at a distant place, having that almanack, were to observe when the moon was at that angular distance from the star, (an observation which he could very readily make, even on ship-board, by means of an instrument called a sextant,) he would know precisely how far the moment when this observation was made, was from the time of the sun's transit at Greenwich; and having observed when the sun's transit took place with him, he would thus, as before, have the difference of longitude.
BOTANICAL RAMBLES IN THE VICINITY OF DOVOR.
No. III. YAVING in my two former rambles examined the country to the east and north of the town of Dovor, I shall in the present one take the reader in quite an opposite direction; and, starting from the Priory, we will first examine the fields which lay to the right towards Buckland, then crossing over to the back of the Heights, we will search that neighbourhood, and then proceed up the valley towards Folkstone, along the high road. Having reached the head of the valley, we will descend the cliffs, and examine Lydden-spout and the under-cliff, returning to Dovor over the downs, via Shakspeare's Cliff
. The fields to the right of the Priory are mostly arable, and will yield us common Fool's-parsley (Æthusa cynapium), Needle-chervil (Scandix pecten), Pansy-violet (Viola tricolor), annual Mercury (Mercurialis annua), and Milk-thistle (Carduus marianus), with its beautifully veined leaves. Crossing from these fields to the back of the Heights, we shall find on the parts of the down adjoining the cultivated ground, some good localities for plants; but we shall only have to notice greater Burnet-saxifrage (Pimpinella magna), and hairy St. John's-wort (Hypericum hirsutum); the former growing in bushy places, on the slope of the Heights, and the latter in and about the hedge-rows, surrounding the cultivated ground.
Returning now to the Folkstone road, we will proceed up the valley, and shall observe as we pass along, stinking Iris, (Iris fætidissima), nettled-leaved Bell-flower (Campanula Trachelium), and common Fleabane (Pulicaria dysenterica), on banks by the road side. The ripe seed-vessel of Iris fætidissima is a very beautiful object. It opens into three divisions, each displaying two rows of brilliant scarlet seeds, and if gathered at the proper time, the seeds will remain on the divisions of the seed-vessel for a long time, and form a pretty ornament in the winter season, when flowers are scarce. In the fields adjoining the road, particularly to the right under some coppices which hang on the brow of the hill, nearly at the top of the valley, we shall find Field Scorpion Grass (Myosotis arvensis), Corn-parsley (Petroselinum segetum), hedge Bastard Stone-parsley (Sison Amomum), red Campion (Lychnis dioica), Henbit-nettle (Lamium amplexicaule), lesser Calamint (Calaminta nepeta), knotted Figwort (Scrophularia nodosa), and bristly Ox-tongue (Helminthia echioides).
About three miles on the Folkstone road we reach the head of the valley, and arrive on very high ground, having to the left a station-house on the edge of the cliff. Here we shall find a circuitous and steep path, which will lead us to the under-cliff surrounding East-Wear Bay, (sometimes called Folkstone under-cliff), and Lydden-spout. As we descend, a beautiful scene presents itself. The under-cliff lies stretched beneath us, extending to Folkstone in one direction, and for some distance towards Dovor in the other, formed of a series of rounded chalk undulations, covered with soft verdure, intermingled here and there with underwood bushes and brambles. A smooth and beautiful beach of sand and shingle bounds it towards the sea, offering a most inviting spot to those who feel inclined to luxuriate in the pleasure of a sea-bath. The cliffs on the land side, not being very perpendicular, are covered in many places with vegetation, and offer a more pleasing outline to the eye than the precipitous cliffs usually seen on this part of the coast. The most beautiful and wooded part of the under-cliff is that towards Dovor, and to this we will pay the most attention, for we shall find more to attract us here; and if we ramble towards Folkstone we shall be straying too far from home.
To the right of the path which leads down the cliff, on some bushy slopes near the bottom of the descent, we shall find large beds of strongscented Lettuce (Lactuca virosa), an useful medicinal plant, but little prepossessing in appearance or odour, the latter being very rank and unpleasant. A thick white juice exudes from any wounded part of the plant, which is very troublesome to those who handle it; it soils cloth very much, and the stain is difficult to remove, as I know from experience. Continuing to the left towards the beach, we shall come to a little stream, running out from beneath a bed of clematis and brambles, close to the sea-shore, where it loses itself among the shingle. Brookweed or Water Pimpernel (Samolus Valerandi), grows near this spot, in damp places; and on the sloping banks on the Folkstone side, we shall observe wild Madder (Rubia peregrina), growing in abundance, and pyramidal Orchis (Orchis pyramidalis). A little further on in the same direction, high up, on a chalk slope, there is quite a bed of smooth Seaheath (Frankenia lævis), which is well worth visiting, to observe this plant growing in luxuriance.
In my first ramble I gave localities for the wild Madder and smooth Sea-heath ; but as both plants are rare, I have mentioned them here again, as they seem to grow more luxuriantly, and be more abundant than on the other side of Dovor.
On and about the sea-shore common Hound's-tongue (Cynoglossum officinale), is to be met with in many places, and the common wild Parsnep is most abundant in this under-cliff, growing in large beds, to the no small satisfaction of myriads of insects, which enjoy the flowers
The reader will find by referring to G. E. Smith's Catalogue of the Plants of South Kent, that several of the Orchis tribe, and other rare plants, are to be found in East-Wear Bay, and at Lydden-spout; but they were all passed when I had the pleasure of visiting this interesting locality.
We will return from the under-cliff by the same way we entered; and when again on the high ground, we will turn our faces towards Dovor, and make our way to Shakspeare's Cliff. In the corn-fields about here, the wild Oat (Avena fatua), abounds, and grows to a very large size. It is a tall and handsome grass, holding its head with pride over the corn it grows amongst. One culm which I gathered was more than five feet high, and the thickest part of it was upwards of a quarter of an inch in diameter. We all associate the idea of Samphire with Shakspeare's Cliff
, from the celebrated lines in King Lear. The plant still grows there in profusion, and the samphire-gatherer carries on his adventurous trade of culling the plant from the surface of the cliff, letting himself down by a rope, which passes over a post, holding one
end in his hands, and having the other tied round his waist. As we descend from the summit of this cliff, on the Dovor side, we shall observe Dyers' Green-weed (Genista tinctoria) growing; and near the bottom of the descent, in the hedges and on the slopes near the beach, we shall find common Alexanders (Smyrnium olusatrum.)
Our rambles in the immediate vicinity of Dovor will conclude here; but I shall, at a future opportunity, request the reader to accompany me in another ramble to Sandwich, where I think we shall find a very ample remuneration for our trouble. Wishing, however, to give as much information as I can on the localities of the plants growing near Dovor, a friend, who collected there this spring, has kindly penned the following observations for me. Most of the specimens I have examined, to
. ascertain their accuracy, and I think the localities may be depended on.
Marsh Speedwell (Veronica scutellata), moist places, East-Wear Bay; wild English Clary, or Sage (Salvia verbenaca), under Shakspeare's Cliff; opposite-leaved Pond-weed (Potamogeton densus,) water in the lane, near Charlton, parallel with the London-road; Corn Gromwell (Lithospermum arvense), close to the London-road, beyond Ewell; common Henbane (Hyoscyamus niger,) among the shingles on the edge of the walk, along the Marine Parade, on the rope-walk, near Shakspeare's Cliff, and again by the Martello towers at Folkstone; Hairy Violet (Viola hirta,) lanes near Guston; common Herb-Paris (Paris quadrifolia,) in a shady lane, about half a mile from the entrance to Lord Guildford's park, on the Dovor side; also abundantly where the wood has been cut down in the same neighbourhood; tuberous Moschatell (Adoxa moschatellina), shady lane, just opposite Kearsney Abbey, near River; Sea Campion (Silene maritima), frequent on the coast; common Columbine (Aquilegia vulgaris) on the confines of a retired wood, about half a mile from the west side of the London-road, beyond Ewell, near Lydden; common Bugle, white variety, lane near Church Hogham; yellow Weasel-snout (Galeobdolon luteum), in shady lanes, near Lord Guildford's Park, also in those beyond River; Hairy Rock-cress (Arabis hirsuta), on the sloping banks as you ascend to the castle; tuberous Orobus (Orobus tuberosus), on the banks of a field, about a mile beyond Kearsney Abbey; Spring Vetch (Vicia lathyroides) dry banks on the London-road, and close to the sea-shore at Folkstone; Green-winged Meadow Orchis (Orchis Morio), in the meadow above the cliff, about half a mile before you reach Folkstone; early purple Orchis (Orchis mascula), in the woods and hedges in every direction, particularly fine in the woods at Waldershare; great brownwinged Orchis (Orchis fusca), on a steep chalky hill, leading from Lydden to Waldershare; Spider Ophrys (Ophrys aranifera), abundant in East-Wear Bay, on the under-cliff and close to the shingles.
W. W. S.
PREVENTION OF EXPLOSION IN STEAM-BOILERS.
ANALYSIS AND CURSORY EXAMINATION OF A REPORT, MADE IN CONSE
QUENCE OF AN INQUIRY INTO THE CAUSES AND MEANS OF PREVEN-
Public attention, in the United States, having been roused in the most painful manner, to the disastrous consequences of the frequent explosions of engine-boilers on board steam-boats in that country, the managers of the Franklin Institute of Pennsylvania determined upon an inquiry into the causes of these dreadful events. Such a philanthropic determination was in perfect harmony with the spirit and conduct of the great man, whose name they have adopted as a distinctive appellation; and if they have not hitherto perfectly succeeded in developing every point of this intricate inquiry, they have the distinguished honour of being the first public or private body which has attempted an examination of the subject, by experiments and apparatus on a large scale, designed on purpose to bring the more important current opinions and theories to the test of actual proof. We think that even their omissions, failures, and mistakes, may be of benefit; they may excite further examination, provoke discussion, and tempt new trains of investigation. The disagreement also of their results, with those of some eminent European experimentalists who have preceded them, will impose the duty of a re-examination upon the latter, in those ins nces, at least, to which the members of the Franklin Institute seem, on their part, to have bestowed the most scrupulous attention.
In a country which, from its hydrographical advantages, is so deeply interested in the extension and prosperity of steam-navigation, it was remarkable, that an inquiry of this importance, rendered imperiously necessary by the repeated destruction of life and property all over the Union, should have been delayed so long. The general government certainly ought to have instituted, much earlier, a minute examination into all the causes of these murderous accidents, and immediately prevented their recurrence, as far as police-regulations could have been effective. They ought then to have gone further, and obtained a carte blanche upon the treasury, to enable them to invite the assistance of the most eminent scientific and practical men, European as well as American, and to direct them to examine the evil in every form, and never to relax their labours until they had discovered such specific means of prevention as would render the most colossal steam-generator as innocuous as a tea-urn. They should not have permitted a self-sustained society to drain its little exchequer, and to call upon a few able men among its members, to devote their leisure hours or sacrifice their professional ones, to undertake a work, which to be effective must necessarily be long, laborious, and costly; demanding considerable scientific attainment, great practical skill, and unremitting attention.
It is not too much to say, that to have completely accomplished this object, would, in the present rapidly and widely-spreading extension of