MANNER OF FLIGHT 109 



of 12 to 15 inches, and an average thickness of i inch. It has a convex 

 and a flat or plane side, and hence falls in the class of shield-shaped 

 meteorites with Algoma and N'Goureyma. Like them, its convex sur- 

 face is the smoother and the flat surface has a scale of oxide best pre- 

 served in the depressions. Winchell does not, however, discuss the 

 manner of flight of the Arlington iron. 



CURVATURE OF DRIFT FURROWS INDICATES L^VO-ROTATION 



The curvature of the radial scorings on the front of the Algoma iron 

 indicates with little doubt that it possessed or acquired a Isevo-rotation 

 about its principal or shorter axis, which, in view of the stability of 

 such a body without rotation may be ascribed to an initial velocity of 

 rotation about this or some near-lying axis when the body arrived at 

 the margin of the aerosphere. At my request. Professor Slichter has 

 investigated this problem from the standpoint of mechanics, and found 

 that a discoid body entering the aerosphere under initial rotation will at 

 once begin a precession and thereafter "go to sleep " after the manner 

 of a top, rotating about its principal (shorter) axis. Thus, initial rotation 

 and forward translation alike require a perpendicular attitude of the 

 disc to the air which it is traversing. Professor Slichter's discussion of 

 this subject follows immediately upon the conclusion of this article. 



PROBABLE RUPTURE OF ALGOMA IRON DURING FLIGHT 



The hackly marginal surfaces of the Algoma iron indicate, it is be- 

 lieved, the remnants of original fracture surfaces brought about by 

 failure (rupture) under tensile stresses. It is inconceivable that they 

 should have suffered so little from the compressed air, which has eroded 

 the front of the meteorite, except on the supposition that they were 

 -produced in a late stage of the flight through the aerosphere. It is also 

 a reasonable supposition that the high velocity of rotation of the body 

 and the decreasing strength in the marginal portions, owing to the ever 

 increasing erosion in those portions^ brought about rupture from the 

 centrifugal force of rotation. It appears to be a soluble problem, though 

 doubtless a difficult one, with this supposition to determine the dimen- 

 sions of the disc in its plane of greatest extension before the peripheral 

 portions were thrown off". 



CA USE OF " THUMB MARKS " 



From the foregoing it is clear that the Algoma meteorite exhibits 

 three distinct surfaces, representing as many periods of formation — the 

 undulating, oxide-covered back, with its shallow thumb marks ; the con- 



