COLORED SIGNALS.—Colored signals have for some time been used, upon French Railroads, as a means ot acquainting the engineers of approa'thing trains with the state ol the track, during the night. Each guardian has three bull's-eye lanterns, one white, one green, and one red. White is significantive of perfect security; the green and red are indicative of danger and the necessity oi precaution. But it seems that this system will have to be abandoned. Many people are incapable of distinguishing one color from another, and accidents have positively happened, by the engineer's seeing the green light through the red, or vice versa, and taking the interposed and mingled colors for white.— Instead of holding up and looking out for " track loosened by rain," or " rails upheaved by the frost," he would count on "perfect security," and dash on into obstructions and devastation. This affection of the eye is denominated by physicians chromatopseudopsii; and the " Moniteur" says it is much more common than is generally supposed. Prevot affirms that one person in twenty has it, and Seebeck that fully five men in forty are afflicted with it. Prof. Thomson, of Glasgowi has for several years declared the employment of colored signals on railroads to be attended with great danger. Green and red are precisely the colors most likely to b" mistaken the one for the other. The same journal mentions a person of its acquaintance who was commissioned by a lady to buy her a green silk at Lyons. On his return he brought her a red one, thinking that he had discharged his duty .with extreme propriety. It says, too, that this incapacity to distinguish colors is frequent even in persons whose professions would seem to render great delicacy of eye necessary; and quotes four well-known painters, three surgeons, two paper-makers, two dyers, a shawl washer, a tailor, and a worker in enamel. Several employees on the Northern Railroad have been convicted of it. Gardeners are often obliged to squeeze an apple or pear to find out whether it is ripe. Its color is no guide. The "Moni-? teur " also knows a stationer who offers you j white sealing-wax for red, and cites the ? case of a manufacturer of paper-hangings who could not tell red from pea-green, and who made so many mistakes that he had to go out of the business. In consideration of all which facts, the Directors of Railroad Companies in France are urged to hit upon a surer method of communication between the guards and the conductors. That there are many persons who cannot distinguish one color Irom another, we have I no doubt, but they cannot be numerous, and they must certainly have a knowledge of their own defects. We do not believe that there is a single painter or dyer so defective in distinguishing colors in the world—not one. It has never been our lot to become acquainted with a single person who could not distinguish colors; it is our opinion, theretore, that colored signals are perfectly safe. COMPRESSED AIR CARRIAGE.—The Paris Presse says that it has seen upon the Champs Elysees, a carriage containing two persons, proceed for twenty minutes, at the full speed of a horse, by means of a cylinder of com-I pressed air, of so small a volume that you might put it in your pocket. The inventor thinks he has obtained a practicable plan ot j utilizing compressed air, and has discovered a means of compressing it at a merely nominal cost. A paper on this subject has been read to the Academy of Sciences. M. Julienne proposes to adapt the principle to carriages and street vehicles merely, to which it :oay be applied with great economy and perfect safety. With a small cylinder, a party may take an airing at the Bois de Boulogne at any rate of speed, irom a walk* up to a gallop. TABLE MOVING IN GERMANY.—It seems | that experiments upon the moving of tables, i as practiced in the United States, are very common in Germany. The " Gazette d' Augsburg " was the first paper that called attention to the phenomenon, and described the manner of producing it. Dr. Loew, of Vienna I who seems to be a man of note, gives an Ik explanation, by experiments upon a light wooden table—persons enough to surround it completely must place their hands upon it, and join them to those oi their neighbors by the extremity of their little fingers. Alter a space of time, varying from halt an hour to an hour and three-quarters, the persons surrounding the table ieel a slight shock; the table seems to dilate and to crack, as if it were standing too near the fire; it then turns half round upon its axis, and starts off toward the north. A person may even withdraw irom the chain and another take his place, without damaging the experiment, if it be quickly done; but everything is lost if a person not in the chain places his hand upon the table. Dr. Loew explains this phenomenon, which he says he has seen produced time and again, by the negative and positive electricity contained in the left and right sides of the human body. When a circular chain thus formed ot persons whose left side touches the right side of their neighbor, and vice versa, acts for a length of time upon a table or any otherbody, this body Would undergo the same change as a bar of iron when placed in the current of induction from the magnet; that is, one of its sides is magnetized positively and the other negatively. The body thus becomes a magnet, and turns upon its axis, till its southern half points toward the pole, and then will continue to advance towards the north as long as its magnetized condition undergoes no modification. INSPECTION OF BOILERS.—In France there has not been one explosion of a steam boiler for every hundred in our country, and it would be well to copy her plan of preventing such disasters. Every steam boiler in that country must be tested to three times the pressure it is intended to bear, and this test is repeated annually. Cylinders oi steam engines are tested in the same manner. FARADAY ON LIGHTNING CONDUCTORS.—On the 22nd of last month (April,) Proi. Faraday delivered a lecture on electricity before the Royal Institution, London, in which he directed attention particularly to those conditions of electric force exhibited in the phenomena of conduction and insulation, and which is of no small interest to all our people, as it relates to lightning rods. He commenced by showing the difference between the conducting powers of metals—iron and copper —and the difference between the travel of heat and electricity through them. The charge of a Leyden jar was sent through a long wire suspended from the top of the theatre to show that no perceptible interval occurs in the transmission of electricity through such a length of wire ; as proved by Mr. Wheatstone, electricity travels at the rate of 300,000 miles in a second. Many electricians suppose that there are conductors and non-conductors of electricity, but this is not altogether correct. The discovery of the conducting and non-conducting properties of the bodies exhibiting this difference were quite distinct, but there are no substances in nature that conduct electricity without offering resistance, whilst the most perfect of what are called non-conduitors, transmit some portion of the electric fluid. Conduction and non-conduction resolves itself entirely into a question of degree. When a prime conductor of an electrical machine was touched with a glass rod, no sensible effect was produced; a walking stick discharged the conductor entirely ; a small copper wire discharged electricity as iast as it was produced in the machine, whilst a dry lath of wood conducted only part of the fluid. By reducing the thickness of the lath its conducting power was diminished, but by shortening the said lath its conducting power was increased. The resistance which the very best conductors offer to the passage oi electricity, has an important practical bearing in reference to the efficacy of lightning rods. It is essential in the construction of lightning conductors that they should be of sufficient size to carry off the lightning to the earth. A French commission had decided that a rod of iron of seven-tenths of an inch square is sufficient. In England the lightning rods fitted to the light-houses were of copper, about one quarter of an inch thick. The capacity of copper to conduct electricity -was seven times greater than that of iron, and yet in last January, al. though the Eddystone Light-house was protected with such a copper rod, it was much damaged with lightning. As a long rod offers more resistance to the passage of electricity than a short one, it is very evident that one which would be perfectly suitable for a house would not be so for a steeple ; the higher the spire or house, the thicker should the conductor be for it. TELEGRAPH FOR PARLIAMENT.—Another application of scientific ingenuity serves to illustrate the various uses of electricity in a rather striking manner. The* House of Commons has set up an electric telegraph of its own, connected, of course, with the telegraph offices in the Strand, by means of which the Honorable members will be able to communicate .Jvith their constituents, touching the divisions and debates, and the progress of nightly legislation in general. The convenience cuts both ways, and the electors on their part will be able to transmit significant hintt to their Representatives of their wishes, and occasionally, perhaps, of their commaidi on any particular question, while a discussion is going on, so that no member will be able to plead ignorance of the feelings of tho"e who sent him to St. Stephen's. A new electric, olock, coupled with an apparatus on the electri cal principle, ior ringing thirty bells, notifying torpfd or absent M. P.'" that a division is at hand, is also a fresh Parliamentary appurtenance. HUMBOLDT ON TABLE MOVING.—The " Si-lesian Gazette " publishes a letter by Alexander Von Humboldt, to a friend who had applied for his opinion upon the supposed mag-netical phenomena of table moving, which has been described in several journals. The veteran physicist remarks that it is always easier to destroy a false theory, than an inaccurately apprehended fact. He then adverts to a long series of pseudo-scientific discoveries, which have been made and exploded in the course of his eighty four years' experience, and advises the table movers to "try their chaff upon some younger bird."
This article was originally published with the title "Foreign Scientific Memoranda"