June, and July have given very accordant results in the hands of Roscoe.-19 C, VII., 202.

NEW METHOD OF MEASURING THE VELOCITY OF LIGHT.

A simple and possibly accurate method of measuring the velocity of light is suggested by Mr. Burgue. A disk, turning very rapidly about its axis, is at each turn to be illuminated by an intermittent and instantaneous light. A single dark radial line on the disk will seem at rest, like the disk itself. Now withdraw the source of light to a distance, and the time the light takes to reach and illuminate the disk will become greater, and the position of the radial line will appear displaced to a new position, forming with its previous position a certain small angle, which will measure the time of the light's passage over a given distance.-1 A, II., 262.

GREAT FRENCH LIGHT-HOUSE AT LA HAVE.

The great French light-house at La Have, near Havre, is said to be the most magnificent establishment of its kind in the world. The electric light was first used at this place in 1863. The electricity is furnished by magneto-electric machines, and the simple uncondensed beam of light is equal to 4000 candles, and it seems to be the testimony of navigators that this electric light is always seen, even in clear weather, before the oil lamps nearer them. Its range of visibility is also correspondingly greater, the radius of the circle lighted up being from five to ten miles greater than that of first-class oil lamps. The difference between the two is, in fact, similar to the comparison of a candle and a gas-light. At some distance there is also a notable difference in their aspects, the electric light appearing white and brilliant, the other red and smoky. The superiority of the former is still more manifest during foggy weather, since at such times, even before perceiving the electric light, its presence is marked by the illumination of the thick atmosphere surrounding it, and its range thus increased. This is an important advantage of the electric light, and may be of great practical utility, as has, in reality, frequently been the case. While its brilliancy gives it this superiority, its inferior power of penetration diminishes the range of the electric

light in foggy weather, and more notably as the fog thickens. In general, it is found that if the electric light has an intensity two and a half times greater than the oil-light, it will penetrate fog as well as the latter. The expense of the powerful electric light at La Have is about one seventh greater than that of the decidedly inferior oil-lights. General testimony seems to be in favor of the introduction of the magneto-electric light at all important stations, and both the English and French are extending its application. Elliot's European Light-house System, p. 248.

THE ROMAN PHAROS IN DOVER CASTLE.

There is still standing within the walls of the castle at Dover, England, an old Roman pharos. The antiquity of this light-house, which has probably not been used as such since the Norman conquest, no doubt exceeds that of any light-house in Great Britain, it having been built, as is supposed, about A.D. 44. Upon it burned for many centuries great fires of wood or coal, the modern system of lamps and reflectors having superseded coal fires during the last century. This pharos, like the one at Boulogne, is built of bricks in color and shape like those found elsewhere in the Roman structures of Great Britain. They are of a light red color, about fourteen inches long, and not more than an inch and a half thick. The mortar joints are of nearly the same thickThe preservation of this famous relic is doubtless due to the fact that some centuries ago the tower was turned into a belfry, and was surrounded by walls of stone. The latter are now nearly destroyed by time, and the old remaining work is again exposed.-Elliot's European Lighthouse System, p. 73.

ness.

THE POWER OF THE ELECTRIC LIGHT.

The most powerful artificial light at present in existence is that employed for the great light-house at Souter Point, on the coast of England, near the mouth of the Tyne.

On both banks of this river there is an immense number of manufactories of all kinds, the smoke from which, under the influence of the west wind, seriously obstructs the approaches from the sea. Fogs at this part of the coast are also frequent; and the problem of light-house illumination

required that such light should be secured as would penetrate through any slight fog or haze, it having been generally acknowledged that not even the sunlight itself can penetrate an ordinary dense fog. The electric light established at Souter Point after condensation is equal in power to 800,000 standard candles, being eight times as powerful as the best American fixed lights. The electric spark passes between slender pencils of carbon, which are themselves consumed at the rate of about one inch per hour. The electric current is generated by two of Professor Holmes's patent rotary magneto-electric machines, driven by steam-engines of six horsepower. The number of revolutions made by each machine. is 400 per minute, and 12,800 sparks pass per minute when both machines are at work. These sparks are, of course, formed so rapidly that the eye does not separate them, and the result is an intense beam of light, so dazzling that the eye of a person within the lantern can not rest upon them for an instant without intense pain. As observed from a distance of several miles, this light is so bright as to cast a well-defined shadow upon the deck of a vessel.-Elliot's European Light-house System, p. 120.

ELECTRIC LIGHT FOR LOCOMOTIVES.

A series of satisfactory experiments has lately been made in Russia in regard to lighting railway tracks from the locomotives by means of the electric light. The track on one occasion, with a battery of forty-eight cells, was brilliantly illuminated 492 yards ahead.-23 A, April 9, 1875, 467.

THE BLACK-BULB-IN-VACUUM THERMOMETER.

As is well known, the black-bulb-vacuum thermometers employed for observing the solar radiation give very discordant results, even in the hands of the best observers, and the origin of this has recently been studied by Mr. Hicks, of London, who states that in his opinion the discordances are in a great measure due to the imperfect vacuum that exists within the inclosing bulb. Having made a large number of thermometers with special care, in which the vacuum has been reduced to the lowest attainable limit, Mr. Hicks finds that it is possible with proper care to always construct instruments that shall be perfectly comparable with each

other. In order that the meteorologist may at any time test the perfection of the vacuum within his tube, Mr. Hicks has very ingeniously inserted two wires into the sides of the bulb in such a way that a galvanic current applied to the wires will, by the nature of the light that is spread through the vacuum bulb, show with considerable accuracy what proportion of gas, and especially of watery vapor, is there present. A pressure within the vacuum bulb exceeding one tenth of an inch of mercury is not admissible if an accurate instrument is desired, and the vacuum can be easily brought to within one fiftieth of an inch, in which condition the radiation solar thermometers will prove strictly comparable. Especially is it important that the bulb should be filled with dry gas, and that not the slightest trace of moisture should exist. Mr. Hicks said that, although he had made hundreds of tubes with Torricellian vacua, he never knew one to fail showing stratification and white light when the tube was thoroughly clean and free from moisture. Quar. Jour. Meteor. Soc., II., April, 1874.

THE THERMAL CONDUCTIVITY OF MERCURY.

Herwig has been continuing the inquiry previously instituted as to whether the thermal conductivity of mercury varies with the temperature-a question of much moment in connection with the reliability of the indications of the mercurial thermometer at different temperatures. He finds that between 40° and 160° Centigrade the heat-conducting power of pure mercury is perfectly constant. He is now occupied in a series of experiments to show how far solid metals differ in their behavior from mercury.-13 A, Feb. 27, 1875, 222.

A NEW SOURCE OF ERROR WITH THE MERCURIAL THER

MOMETER.

Mr. J. M. Morgan, in employing a mercurial thermometer in the operation of distillation, the instrument being inserted into the apparatus to such a depth that the whole of the quicksilver thread was surrounded by heated vapors, observed after the operation had continued for several days that the temperature registered was too low by 3°. An examination showed that this error was due to the fact that a portion of the mercury had vaporized and condensed in

the upper part of the tube which was not exposed to the heat of the operation. When the portion condensed in the upper end of the tube was united, by cautious tapping, with the main column, the instrument again registered correctly. The observer of this phenomenon determined thereupon by experiment that a quantity of mercury corresponding to from 1° to 15° will be volatilized in the manner described if the mercury column of a thermometer is exposed for several days to a temperature of from 60° to 100° Centigrade. This observation is worthy of the special attention of experimenters, since the small quantity of mercury thus condensed in the upper end of the thermometer tube may be readily overlooked, and thus give rise to serious errors of observation.-Fresenius's Zeitsch. für Analyt. Chem., XIV., 81.

RELIABILITY OF SIEMENS'S PYROMETER.

The Siemens pyrometer has been subject to a careful investigation by a committee of the British Association, in order to decide whether or not the resistance is altered after exposure to high temperatures. Four instruments were examined, three of which were found to be considerably altered after having been exposed to a high temperature; the fourth gave results showing it to be sufficient for industrial application, if not for strictly scientific observations.-12 A, X., 373.

A NEW MERCURIAL MINIMUM AND MAXIMUM THERMOMETER. Mr. Denton describes a maximum and minimum thermometer combined in one, by which both registrations of temperature are obtained from one mercurial bulb, both indices are moved by the mercury pressing on their ends, and, independent of the self-registering feature, the actual temperature is shown, at any moment, by two separate columns of mercury. In the construction of the instrument the tube of the maximum thermometer is bent at the top and turned downward, and dips into an hermetically sealed chamber, which is itself more than half filled with mercury. An increase of temperature raises the index of the maximum thermometer and pushes down the mercury in the other leg of the tube. A diminution of temperature leaves the maximum index in its place, and allows the mercury in the other leg