fig. 1.* As we come further south, the moon will appear higher; and on the line from Malin Head, in Donegal, to Wooler, in Northumberland, the centres of the two discs will coincide so as to exhibit a regular ring of light as in fig. 2. When we reach the line from the south of Donegal Bay to Whitby, the moon will have ascended so as just to touch the upper part of the sun's disc, as in fig. 3. The unobscured part will be still greater when we reach the latitude of London, but even there the darkness will extend over very nearly ths of the sun's diameter, as represented in fig. 4.

At the height of the eclipse a remarkable appearance will occur, which may be worth observing. The light which passes directly from the sun through any small openings, will form the images of luminous crescents. This depends upon a very common and well-known optical phenomenon. If a small hole be made through the window shutter, an inverted picture of all external objects is formed on the opposite wall of a darkened room. Now suppose this aperture to be a physical point, the rays coming from the sun, all converging to this point, will form a cone of light on the outside. A similar cone will be formed on the inside by the same rays after they have crossed at the aperture; and, consequently, if a screen be held perpendicularly to them, a circular luminous

The crescents of light are drawn without any inclination, because the angles which they will make with the vertical, will vary in different places.

Eclipse begins
Greatest Phase
Ends

1 h. 51.2 min. May 15 d.

19.1
39.1

Greenwich Mean Time.

image will be formed on it. Let now a second physical point be situated close to the first, then another similar cone of light will be formed, so that the two, as they gradually enlarge, will gradually overlap each other. As the greatest distance of the two circumferences can never in any part exceed the distance of the two points from each other, the circles, when much extended, will no longer be distinguishable from each other, but will perceptibly form one round image. With a third, fourth, fifth point, the same effect will be produced; and at their separation from each other will not make any difference, it follows that if the aperture is of any figure, however irregular, the image will resemble it when the screen is near, but as the screen is removed, the angular parts at the edges of the image will be gradually rounded off till the outline becomes to the eye a regular curve. The experiment has hitherto been described as it usually takes place, but it must be clear that the roundness of the image depends upon the sun's full orb being circular; when the illuminated disc is reduced by the eclipse to a crescent, the image also will be formed of a similar figure. This will be curiously seen in the shade of a tree, between the leaves of which the sun's light may pass.

ON SCREENING A LIGHTITOUSE. By Lieut. Raper, R.N.

To the Editor of the Nautical Magazine.

SIR. In conversation with the hydrographer to the Admiralty, concerning the distance at which the screen interposed between the light of a lighthouse, and a danger to be avoided, should be placed from the light itself, it occurred to me that the question was capable of a very simple and concise investigation, which is here offered for the perusal of your readers, as matters relating to lighthouses are of consequence to the nautical world.

ness.

Since the light of a lighthouse is not a mere luminous point, but exhibits to the eye a visible diameter, the shadow cast by the screen is not strictly defined at the edges, but has a penumbra extending to a certain distance on both sides of the complete darkSince then the vessel can in no case lose sight of the light instantaneously, the object is to determine the distance of the screen, so that the change in the appearance of the light in a small interval of time, to a vessel in the neighbourhood of the danger, may be so distinct as not to be confounded with a gradual obscuration caused by the state of the atmosphere, nor to have any doubt that its total disappearance is owing to her having entered the forbidden ground.

R

H

K

Suppose AB the diameter of the light, CD the screen, R a rock; a vessel standing towards R across the line joining the rock and lighthouse, enters the penumbra at н, on crossing the line ADH, and the complete darkness at K, on crossing the line BDK; hence the light is gradually disappearing while she is going along HK.

Now the diameter AB of the light, and the distance of the rock being given in any proposed case, the line нк can be diminished only by moving CD along BK towards K, or from the light; that is, the distance Hк depends on вD, or, which is the same thing, BD on HK, HK being the distance run by the vessel, must be assumed, and this assumption of distance must depend on the velocity which we assign to the vessel, and the time allowed for perceiving a visible change in the light; therefore кH, and consequently BD, or the solution of the question, depend entirely on practical considerations. We shall first get an expression for BD, and then make certain assumptions to adapt it to practice.

We may suppose, for simplicity, that the brilliancy of the light, or the quantity of light which strikes the eye, is proportional to the portion exposed by the vertical edge of the screen, and its decrease proportional to the quantity concealed by the edge of the screen in the same way. Also, we may suppose that a change in a light dimmed by distance and haze, which shall be conspicuous, cannot be less than one-third of the whole quantity of light. Take A a equal to one-third of AB, and draw a Dh, нh will be one-third of HK. Now HK being small with respect to the whole distance BK, the angle KDII, and its equal ADB, will be small, and these angles will be therefore proportional to the lines which subtend them. Then, since ADB➡ KDH, we have

or, since AD and BD are nearly equal, and Dк nearly equal to BK,

or, putting a for BD, the distance of the screen required in feet, I for AB, the diameter of the light in feet and decimals; a for BK in miles; and tv for the space нh, t being the time in fractions of an hour, and v the velocity in miles, or rate per hour, then

x = }

al
tv

which is the formula for general reference.

Assume now that the vessel is going four knots, and that five minutes is the time allowed for perceiving the above change in the appearance of the light, by the loss of one-third of the whole quantity of light; and suppose, for example, that the diameter A B (or 1) is 1.4 feet, and BK (or a) 7 miles; then

x = }

1.4 X 7
X4

= }

9.8 × 12
4

9.8 feet, the distance required.

But a vessel will not, except when working to windward, cross the line BK with this velocity; hence two knots is perhaps as much as, or even more than, we ought to allow; moreover, five minutes is a long time to strain the eye in watching a light, and two minutes and a half would be a better assumption. We shall now have

The screen therefore should not be put nearer than forty feet in this case, where the light is 1-4 feet diameter, and the danger seven miles distant.

HYDROGRAPHICAL NOTES MADE IN THE GULF OF CALIFORNIA. By Mr. R. C. Allan, late of H.M.S. Conway.

Passage from San Blas to Mazatlan.

THE direct course and distance from Piedra Blanca de la Mar (which is 12 miles to the westward of San Blas) to Mazatlan, is N. 30° W. (true) 110 miles. The Conway was four days making this passage. The wind, for the most part, was light breezes from N.W., (which is the prevailing wind from the gulf,) with fine weather, and we experienced a current of about half a mile per hour to the southward. In making this passage, it is perhaps the best way to stand off, or to the westward on the starboard tack during the early part of the day, and in again in the latter part, as we found the wind hauled round more to the westward in the afternoon.

Passage from Mazatlan to Guaymas.

The average passage from Mazatlan to Guaymas is 10 days; the Conway did it in 8 days.

Tuesday, January 6th, 1835.-Sailed from Mazatlan at 8 p.m.

Saturday, 10th.-Made Farallon de San Ignacio, (a small rocky island about a mile and a half long,) having worked up on the east side of the gulf. Moderate winds and fine weather.

Sunday, 11th.-Wind fresh from N.N.W. Stood over to the western side of the gulf, and tacked off Isle St. Josè, which is about 15 miles in length. To the northward of St. Josè are two small islands, named Santa Crux and Don Diego: the former is about five miles in length, the latter two. The main land is high hereabout, and is a strange mixture of rugged hills and tableland. About 8 miles to the eastward of the north end of St. Josè lies a small white rocky islet, which is not inserted in the Spanish charts. A rocky ledge between St. Josè and the main land, and another between Santa Cruz and the main, were seen, but the ship was not sufficiently near to judge of their extent.

Stood

Monday, 12th.-Wind fresh from N.W. to W.N.W. over to the eastern shore, and tacked off some very low land, situated in lat. 26° 52′ N. and long. W. (of San Blas 4° 16′ 33′′) of Greenwich 109° 35'; no high land was to be seen behind it, but some very high in the E.N.E. Vessels must be cautious how they stand in here in the night time or in thick weather.

Tuesday 13th.-Light winds towards the evening. Stretched over to the Californian shore. The breadth of the gulf between the parallels of 26° and 27° is about 76 miles.

Wednesday, 14th.-Light winds from the S.E. with thick misty weather at noon it cleared off, and we arrived at Guaymas at 5h. p.m.

CURRENTS OF THE OCEAN.

Extract of a Letter from B. A. Ancell, Esq. to the Secretary of the Admiralty.

Porlock, 14th April, 1836.

THE enclosed paper was picked up yesterday floating near this shore, in a wine bottle, at the bottom of which (inside) was a small quantity of lead sufficient to preserve its equilibrium, and to prevent the wind from having any influence. On the exterior, round the shoulder and the bottom, was a quantity of barnacles advanced in growth, which assisted in increasing its draught of water to nearly as high as the mouth, which was carefully stopped with cork and sealing-wax.

"Barque Sarah, of Newcastle, Capt. James Mickle, of Newcastle, Master, and J. Weatherley, of N. C., passenger.-This bottle, thrown overboard off the banks of Newfoundland, 29th May, 1835, in lat. 46° 2′ N. and long. 48° 10′ W. On outward-bound passage from Newcastle to Quebec."

The supposed track of this bottle completely coincides with those of three others from nearly the same latitude and longitude, and clearly shews that the surface water from the banks of Newfoundland eventually reaches the western shores of Great Britain. NO. 51.-VOL. v.

2 P