(12551) W. E. W. Y. says: On the evening of September 7th, at 7 :30 o'clock, there appeared a beautiful lunar rainbow in the western heavens. At about 7 P. M. a dark cloud arose in the west, and rain fell heavily for some minutes. It was perfectly clear in the east, and the full moon, coming up from the under world, shot its rays through the rain drops, with the result of a gorgeous rainbow at night. This is the first time a phenomenon of the sort has been observed here, so far as investigation discloses. Are there many instances of lunar rainbows on record? A. Lunar rainbows are not a frequent occurrence. This one seems to have been unusually brilliant. Generally but a few colors are seen. The moon on September 7th was within one day of the full, and hence gave almost the maximum of light. This would account for the brightness of this lunar bow. The noted geologist, Edward Hitchcock, formerly president of Amherst College, Massachusetts, once said in an address, “He who has seen one total eclipse of the sun, or one transit of a planet over the sun, or one November shower of meteors, or one splendid comet, or one lunar iris, or one volcanic eruption, may be satisfied, and cannot hope for a second sight.” The writer of this note has seen all of these grand phenomena of nature, excepting a meteor shower of the first rank, and a great volcanic eruption; but he has had the privilege of seeing a lunar iris on two occasions, and ha s seen volcanoes in action and many smaller meteor showers. The geologist was right in his list of rare natural phenomena. (12552) F. R. K says: I desire to copper-plate glass plates, 20 inches by 26 inches, for use as a condenser in wireless telegraphy. Will you please give some method of coating the glass to render it a conductor, and also give voltage necessary? A. Glass may be prepared for copper,plating by giving it a coat of copal varnish or of gutta percha in benzole. When this ,is .dry, it will take plumbago in the ordinary way. Copper-plating with an acid bath requires from 0.5 to 1.5 volts. With a cyanide .bath, 2 to 5 volts are needed. Various processes and formulas may be found in Watt's “Electro-Plating,” price $4.50. It would seem necessary to have some good manual at hand for the numerous details of the work. It would be much easier for a novice to coat the glass with silver, as in a mirror. This process is fully described in our Supplement 1671, price ten cents. The silver surface would be just as effective as a copper coating. The silver could be plated with copper. It might be the easiest way to obtain a base for the copper plating to deposit a thin coating of silver on the glass and plate the copper upon that. (12553) W. A. S. asks: Have you a Supplement that will give detailed information in regard to magnetizing magnets, such as are commonly used on such magnetos as are used for gas engine ignition? A. Unless one has a powerful direct current of electricity at his disposal, it would be better to send magnets away to be magnetized. The process. however, is simple. A coil of wire should he made of such size that it will slip over the magnets, so that the magnets may be passed through the coil. No. 16 B.&S. single cotton-covered copper magnet wire may be used, and 100 turns will be sufficient. Wind as compactly as possible and in four layers, This will make a coil about an inch and a half long' and less than a half inch deep. If made for permanent use it should be well filled with shellac and dried before taking it from the form or spool upon which it has been wound. It should also be secured from unwinding by binding it with fine cord. This coil will stand ten amperes or more for some little time without overheating. F'or most convenient use it may be connected in .series with an arc light as a resistance. It cannot be connected directly to a lighting circuit; its resistance is too low. It would be made very hot in a few seconds, or the fuses would be melted out, Pass the magnet to be charged through this coil several times. A small compass will give you the polarity of the coil and magnet. (12554) A. D. asks: Can you give me a formula to make a solution to do electro silver plating with wet batteries 'I A. A solution for silver plating may he made to contain 3 ounces of silver chloride and 9 to 12 ounces of potassium cyanide per gallon of water. The mode of preparation and the care of the bath are fully given in Van Horne's “Modern Electroplating,” which we will send for $1. It is quite as important to keep the bath- in good condition as it is to have it right at the outset.