One more illustration on the subject of gravity, and we leave that branch of physics for the present, not that there is any lack of illustration, but because there are so many other subjects which we have to place before our juvenile readers. The above little figure may be made to balance itself amusingly. Get a piece of wood about two inches long ; cut one end of it into the form of a man's head and shoulders, and let the other end taper off to a fine point. Next, furnish the little gentleman with a pair of wafters, shaped like oars, instead of arms ; but they must be more than double the length of his body ; stick them in his shoulders, and he is complete. When you place him on the tip of your finger, if you have taken care to make the point exactly in the center, he will stand upright, as seen in the engraving. By blowing on the wafters he may be made to turn round very quickly. It is explained by the reasons that were given in our last number. This little machine consists of three vanes, the form of each being the segment of a circle, the obliquity of whose surface increases as it recedes from the center of motion. The flyers are attached to a spindle, which fits loosely in the stand, and around it is wouiid a string, like that around a humming-top. If this string be suddenly pulled, a powerful rotary motion will be given to the vanes or flyers, and from being set at an angle they will cut the air obliquely, consequently a reaction takes place ; and if the weight of the flyers and spindle be less (as it should be) than the force of the reaction, it ascends, whirling its way upwards, like a bird, into the sky. If it be made to spin in a room, it rises to the ceiling, and spins around on the ceiling for some time. It is a highly amusing and instructive toy, as it is on the same principle that birds fly ; or perhaps its action is more like that of the screw propeller in the water, and it might justly be called an :erial propeller. If we are ever able to travel in the air by means of balloons, it will be by the aid of some such contrivance as tliis, although we think that that day is very far distan