keep your Queries on separate sheets of paper when corresponding about such matters as subscriptions. books, etc. This will greatly facilitate answering your questions, as in many cases have to be referred to experts. The full name and address should be given on every sheet. No attention will be paid to unsigned queries. Full hints to correspondents are printed from time to time and will be mailed on request. (12558) R. A. S. says: 1. What is the temperature at which glass and quartz melt ? A. Glass gradually softens as it is heated until further heating produces no further change. It does not become a freely flowing fluid at any temperature, but only a thick, viscid substance which flows like cold molasses. Different kinds of glass begin to soften at widely different temperatures. Any glass may be heated probably 400 deg. to 600 deg. If. without becoming soft. Quartz is melted only by tie electric are, which has a temperature above 6,300 deg. F. 2. What is the temperature at which mercury vaporizes? A. Mercury boils at 674.6 deg. F. 3. What is the average amperage per cell of a storage battery when fully charged? A. The number of ampere hours of a battery is usually given by the surface or weight of plates. It differs for different makes of battery and rates of discharge and size of cell. A storage battery in a certain electric -.:ght station gave this report : 156 cells, 25 positive and 25 negative plates each, at 10-hour discharge rate gave 278 horse-power per hour, average volts 1.9 per cell, 204 pounds per horse-power hour. At one-hour discharge rate, it gave 1,120 horse-power average volts, 1.8 per cell, 506 pounds per horse-power hour. A rule for lead cells has been given: 53 ounces lead peroxide for a 10-hour discharge rate, increasing to 1 ounce for 1-hour discharge rate. The makers of any particular form of cell will give figures of its performance upon application. 4. What is the formula for finding the wattage of a direct-current genertor ? A. The volts given by a dynamo are determined by the rate at which the lines of force in the field are cut by the coils of the armature, and the amperes by the resistance of the dynamo as a whole. The watts are the product of the volts and amperes. These quantities are measured by the instruments in the test of a machine. In designing they' are assumed, and the machine is built to give its rated output. The sizes of wire to be used are selected to carry the amperes, without heating above the safe limit. 5. Will the color of the tube in J- a mercury vapor lamp have any effect on the light '! What is the best color for ordinary lighting purposes? A. The color of the glass in the mercury vapor lamp will modify the light by absorption. The color to which our eyes are accustomed is that given by the sun, a light containing all the wave lengths from red to violet. 6. In a mercury arc in a vessel the size of a 50-ampere tungsten, what amperage will vaporize enough mercury to break the bulb at 110 volts ? A. The pressure of gas or vapor which will break a given bulb varies with the kind and thickness of the glass. No numerical answer can be to the question as you put it. (12559) H. C. says: 1. What is meant by the center of the force of gravity, and where is this point situated in the east? It would seem that if the pull of gravity is toward the greater mass; at the center of the earth the pull will be outward in every direction, and consequently the earth hollow. A. The center of the force of gravity upon any body is the point in the body about which the whole bod is balanced. In a uniform sphere it is at center of the sphere. It is what we call the center of the earth. All the mattel' in the earth gravitates toward that point, all attraction is inward toward that poit, not outward from that point. If a ball be dropped upon the earth and move .ito and through the material of the earth. i.t would drop to the center of gravity of the earth. 2. How is it proved that a plumb bob is deflected 'from plumb by a mass, such as a range of moun.tains ? A. It is proved a plumb line is drawn out of the by a heavy mass, such as a range of mountains, by means of an engineer's instrument for determining a level line and a vertical line. An engineer's transit is such an instrument, but much more accurate instruments are used by astronomers in solving this very difficult problem. When an accurate vertical line is determined. the deviation of the plumb was seen and meas-ul'ed. It has been done a sufficient number of tmes that we may rely upon the results obtained by astronomers. (12560 ) C. C. asks : A newspaper recently gave the velocity of electricity, 304,-r>2:i,1(i() yards per second, and of light, 328,028,800 yards per second. That has given rise to a good deal of discussion among draftsmen and estimators employed here. You would earn the gratitude of some forty or fifty of the men interested. if you would state whether or not the velocities given are correct, and if not. what the correct figures are. A. The trouble with the figures which you quote for the velocity of electricity and of light Is that electricity cannot be said to have any velocity, and the velocity of light is not known with a limit of exactness of 100 yards per second. The l a test determination of th e velocity of light by Prof. Michelson of the University of Chicago is 299.860 kilometers per second, with a possible error of 30 kilometers per second. This in miles per second is 186,330, with a possible error of about 20 miles. The velocity of the electric current varies with the conditions under which it flows. Electric waves in wireless telegraphy move with the same velocity as light.