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(12565) L. G. asks: Kindly inform me if rocks, stones, and boulders grow aK they lie and are in the earth. A. Hocks such as boulders do not increase in size as they lic upon the earth or buried in it. Layers of rocks have formed from their materials being pressed together under the ground by the weigbt of the earth above them in the long ages of the past. (12566) A. G. asks: I am ashamed to ask you, but can yon tell me tl", speed at which air at 60 degI'es and 15 pounds would rush into a perfect vacuum? I cannot fnd the information in two encyclopedias nor t\O books on physics. A. You need not be ashamed to ask at what velocity air will rush into a vacuum, since no one can answer it. Air at 15 pounds pressure rushing into a place where there is no pressure will at that instant have an infinite velocity, the ratio of 15 to O. From that instant the velocity will diminish until the pressure inside is eqnal to that outside, and the velocity will then become zero. Between these two velocities there will be all possible velocitIes. No single velocity can be given. The 60 degrees does not affect the result. If any text-book or eneycIopeLia ga vo a single velecity for this case, it would have to be marked “Interesting, if true." (12567) C. A. P. asks: Please answer the following in your coluln for queries : Does the top of a wagon wheel travel faster than the bottom? Please give explanation. A. The top of a wagon wheel travels along tbe road with the same spcpd as the \agon to wbich it is attached, else it would come off the wagon and go into the ditch. AI! parts ot the wheel also rotate around the axle with the same speed, else the wheel would break into pieces. Lastly, tbe motion of a wheel may be referred to the ground over which it moveB. It will then be found to have a very curious motion. Mark a spot on the felloe with a chalk mark, so that you can see just where it goes with reference to the ground. As the wheel rolls along, the chalk mark coming down to the ground, is in contact with a point on the ground for an instant, and rises again till it is at the top of the wheel, and this it does every tire the wheel moves as far as its own circumference. When the mark is half way down from the top it is in front of thc hub, and when it is half way up again it is hdtind the hub. It has gone back the whole diameter of the wheel, just as it went up the whole diameter of the wheel. It moves down and up, forward and backward, as it rolls around its huh, and nlOves along with the wagon as it whole. The fact is, if the wheel does not lip, the mark stands still on the ground while t is in contact \ith th(' ground. rehe fact also is that the mark at the top of the wbeel moves forward t\icc as fast :R the hub moves. The mark moves backward, as seen from the wagon, from the tim( it is on the level of the hub moving down till it is on the level of the hub going up, and forward during the rest of .ts rotation. If you would see tbc aetual path of a point in tbe circumference of a roil-ng wheel, take a eilcular piece of board and :asten a crayon into a hole at its edge ; then :oll the board along the chalk trough on a school blackboard in such a wa. that the crayon may trace its path on the blackboard. You will have the curve described h. the point of the wheel along a vertical surface, just as the wheel traced its curve in the air. This curve is caIlfd in mathematics a cycloid. You will doubtless find a picture of it in any large dictionary. ,his lnntt
