114 W. H. HOBBS METEORITE FROM ALGOMA, WISCONSIN 



meteorite moving in the atmosphere (which is, of course, a viscous gas 

 and not a " perfect liquid "), but these numbers will suffice for the 

 present argument. Since the momentum of the body is applied at the 

 center of mass 0, while the resultant pressure is applied at the center 

 of pressure C, the direction and magnitude of the resultant couple are 

 dependent on the relative position of these two points. In a symmetrical 

 body, as represented in figure 1, the direction of the couple will be as 

 above described. If the body be irregular in shape, so that the center 

 of mass is farther than three-eighths of the semi-diameter from the 

 geometrical center of the body (as for example in a body shaped like a 

 tadpole), then the couple in question will act in the opposite sense and 

 will tend to turn the body into the air stream and not abreast of it. 



The effect of the accelerating couple just described on the motion of 

 the rotating meteorite is easily ascertained. The result is identical with 

 the influence of gravity on a spinning top or gyroscope, a result com- 

 monly designated as " precession." By the well known laws of rotating 

 bodies, an angular acceleration applied about an axis intersecting at 

 right angles the axis of rotation sets the spinning axis of the body in 

 a new position in the plane of the two axes and tow^ard the axis of 

 acceleration. If the angular acceleration be continually applied (as in 

 the case of a common top), the spinning axis will describe a cone, called 

 ' the cone of precession. The rotating meteorite, upon striking the aero- 

 sphere, must therefore have taken up, as it progressed along its path, a 

 motion of precession entirely similar to that seen in a common top. 

 The rate of precession would be dependent both upon the velocity of 

 translation and the rate of rotation, increasing with the former and de- 

 creasing with the latter magnitude. 



Following the inauguration of the precession, the next phenomenon 

 in the meteorite's motion would be a continuous and rapid lessening of 

 the angle of the cone described by the precessing axis. This phenomenon 

 s also readily explained by the properties of a common spinning top. 

 If in the case of a top or gyroscope we artificially hasten the precession, 

 the top will rise toward a vertical position; if w^e slow the precession, 

 the top will fall. This principle is really no different from that which 

 explains the motion of precession. In each case an accelerating impulse 

 applied to a rotating body with one point fixed, but otherwise free, pro- 

 duces a motion at right angles to the direction of the impulse. 



In the case of a common top the explanation of the gradual rise of the 

 axis of the top toward the vertical (the so-called "sleeping" of the top) 

 may be readily inferred from figure 5, A. This figure represents on a 

 much enlarged scale the blunt point of the top in contact with floor, the 



