Imatges de pÓgina
PDF
EPUB

ticity, would always shoot up through the air to where it would form a cloud, did the temperature decrease as fast as some suppose it to do. It is true, that transparency might still be aided by rapidly descending currents; and it may be partly owing to these, that portions of the sky become visible when the air is loaded with moisture. I am also aware that, in all this, allowance should be made for the different and perpetually varying conditions of the several strata composing the atmosphere.

A very probable reason why the temperature at the tops of mountains is generally found to be lower than that of air at the same height over the plains, is, that mountains are cooled by, and enveloped in, recently dilated ascending currents of air rolling over them. Snow-clad mountains are, besides, cooled, particularly in dry weather, by the evaporation from the snow. And we may remark, by-the-bye, that, at the same temperature, dry air is, for the above reason, less efficacious in melting snow than moist. Because, the moist air, in place of spending its heat to form vapour, does, in consequence of its touching a colder body, part with a portion of the latent heat of the vapour it already contained, which greatly aids in liquefying the snow. The melting of snow does not, therefore, depend solely on the temperature of the wind, but likewise upon its being previously charged with moisture. When dry air passes over snow on a high mountain, the evaporation, and consequently the reduction of temperature will be greatly promoted by the diminished pressure which obtains at such heights. This may help to explain why snow winds, as they are called, should be found so intensely cold. The mere circumstance of wind having passed over a cold mountain, is not a sufficient reason why it should be cold after its descent.

The rise of water, in the water-spout, has been long accounted for in a very rational way. The collision of currents of air from different quarters produce a whirlwind. The air near the axis of rotation is rarefied by the centrifugal force. The pressure on the spot under this attenuated air is necessarily diminished; and, of course, when a whirlwind occurs on the sea, &c., the water rises in the axis, on the same principle as in the common pump. But the rarefied air itself ascends, in virtue of its

levity. Its place is supplied by the concourse of the heavier adjacent air, which, in its turn, is rarefied; and, in this manner, an upward current of air is produced, which aids the ascent of the water. Thus far, the explanation is very satisfactory. The other principal part of the phenomenon, the apparent descent of a dense stem from the clouds, nearly over the spot where the water rises, has been ascribed to electricity. But this, in my humble opinion, is rather an evasion than an explanation, and seems nearly allied to the notion that hills attract clouds.

The column which apparently descends from the clouds (for any descent is altogether illusory till water actually fall) may be accounted for in the same way as the mountain-cap, viz. that it is moisture condensed by the cold due to the rarefaction which is occasioned both by the whirling motion of the air and by its rapid ascent. The more rapid the rotation, the greater will be the rarefaction and cold; and, of course, the lower down will the condensation take place. The sound and flashes of light seem to be thunder in miniature.

In attending to this and other atmospherical phenomena, most people are embarrassed with a preconception which is not easily overcome that clouds are solids, or something much more substantial than the air in which they are formed.

Volta supposed that bodies, while passing into the gaseous form, absorb electricity, which they emit again on being condensed. Objections have been made to this theory; but as they rest, in a great measure, on the assumption that we possess perfect electrometers, they do not convince me that the theory is unfounded. Thunder is unknown in the Polar regions, and rarely takes place any where in cold weather. From which it appears, that thunder does not occur in air incapable of containing much moisture; as is further confirmed by its being ordinarily produced in a dense cloud. The conjecture which I have to throw out is, that when a large mass of warm damp air is suddenly moved upward, it dilates, is cooled, and deposits a considerable share of its moisture, which, in laying aside the gaseous form, parts with electricity and emits lightning. The sound may be partly a tremor, which the air sustains at the moment the pressure is relaxed by the vapour

losing the elastic form, and may be partly an effect of the electricity in making its escape from the cloud. The thunder and lightning which sometimes attend the condensation of large volumes of steam emitted by volcanoes, are favourable to this theory; as are likewise the noise and lightning of the water-spout, if not some parts of the Northern Lights.

Remarks on Captain Pakenham's Temporary Rudder, and explanatory of an Alteration proposed. By Commander JOHN PEARSE, R. N.

HAVING twice witnessed the application of that most useful invention, it has frequently occurred to me that it may be made much more effective, and the labour and time in making reduced, without lumbering a ship with any additional materials, except something to secure the heel of it in place of the spare lower cap.

The rudder being such an essential part, and requiring to be replaced as expeditiously as possible, in the event of its being damaged or lost by a ship getting on shore, or, which sometimes happens, at sea; and, on such occasions, ships having to depend on their own resources, particularly on long voyages, has induced me to offer the following remarks on it, and an explanation of an alteration I propose.

The top-rope sheave holes being necessarily cut in the spare topmasts, for the expedition of shifting in case of emergency, the mast is so much weakened by it where the most strength is required, when applied as the main piece of a temporary rudder, and twists so much when the ship carries a weather helm, that it frequently becomes necessary to ease it as much as possible by furling the after sails. This defect may be remedied by supplying one piece of pine timber of the length, and in lieu of the two yard-arm pieces which are given to all king's ships as

sea store.

Figure 1st, in the accompanying wood-cut, represents the temporary rudder agreeable to Captain Pakenham's plan. By which it will appear evident, that the rudder is considerably weakened by the sheave hole where the greatest strength is required. The diameter of the sheave hole A is one

[ocr errors]

sixth of the diameter of the mast, and which, from the sheavehole being through its centre, is reduced in strength and substance full one-fifth.

It also requires considerable time and labour to convert the spare lower cap, to adapt it to the purpose of securing the heel. The wood of the aft-part, at B. fig. 2. requiring to be cut out

[graphic]

Fig. 2.

D

[ocr errors]
[ocr errors]
[ocr errors]
[ocr errors]
[ocr errors]
[ocr errors]
[ocr errors]
[ocr errors]
[ocr errors]
[ocr errors]
[ocr errors]
[ocr errors]
[ocr errors]
[ocr errors]
[ocr errors]
[ocr errors]
[ocr errors]
[ocr errors]
[ocr errors][ocr errors]

TOP

Idiom ni s

qule snel

inst nor to ar

to admit of the cap fitting on to the stern-post, and consequently the brace bolts at C. to be driven out, or cut through

with a hack saw; the brace bolts at the fore-part also require to be cut, to admit of the corners being rounded off. The bulk of the cap must also contribute to the tendency of the rudder to lift, by the sending motion of the ship in a sea, except it is fitted close under one of the braces, which is not practicable if the pintles are broken and remain in, as is generally the case when a rudder is lost.

From the buoyancy of the materials the rudder is composed of, it is also necessary to append iron ballast to the heel, to sink and facilitate the hanging of it; all of which would be greatly remedied by a cast composition cap, as described in fig. 3. being supplied for the purpose. As its weight would obviate the necessity of appending ballast to the heel, its small bulk would add but little to the tendency of the rudder to lift in a sea; and the time and labour of converting the spare lower cap saved.

The greatest difficulty in hanging it, particularly if there is any motion, is, to conduct the cap to its place. From the position of the eye bolts D, in the spare lower cap, the hawsers which are fastened to them, and which secure it to the stern-post, can be no guide to lead it to its place, consequently it is necessary to have guys to assist it; and which would be remedied by the composition cap I propose, as the hawsers being fastened to the eyes EE would lead it immediately to its place, and obviate the necessity of having so many ropes, which are liable to confuse and get foul of each other. -Line-of-battle ships are allowed only two spare pintles as I have represented in fig. 1. I should propose these being supplied, as it would make the rudder much more secure; and the third brace down, not being much below the surface of the water, can easily be cleared of the pintle if broken and remaining in.

In an eighteen gun-brig, which I was serving as first lieutenant, the temporary rudder was put together and hung in twelve hours. By the alteration I propose, and the superior resources of a large ship, it could no doubt be done in much less time; and it is a much more expeditious means of rendering a ship effective than repairing the pintles, when they are broke and the rudder saved. Accidents, too, may occur, when and where it is

« AnteriorContinua »