TRANSACTIONS OP THE SECTIONS. 3^ 



from collapsing inwards. The nearly spherical shape of the shell would also greatly 

 increase its resistance to forces acting perpendicularly to its surface, so as to cause 

 parts to subside, while the action of elevatory forces would pot be resisted in the same 

 manner. 



On the Eclipse of the iSun mentioned in the First Book of Herodotus, 

 By the Rev. Dr. Edward Hincks. 



The author maintained that the eclipse of the 18th of May, 603 B.C., was tha* 

 which terminated the Lydian war, and that from this celebrated eclipse and his 

 knowledge of the period of 223 lunations, Thales had predicted the eclipse of the 

 28th of May, 585 b.c. Herodotus, he thought, had confounded the two eclipses 

 with which the name of Thales was connected. 



Previously to the publication of Mr, Baily's paper in 1811, it was generally believed 

 by astronomers that the eclipse of 603 b.c. satisfied the conditions of that which 

 terminated the war, the field of battle being supposed to be in the neighbourhood of 

 Kars. Now that Mr. Baily's arguments against this eclipse have been shown to be 

 erroneous, the author regretted that recent writers had neglected it ; the elements of 

 it having never been calculated with the improved lunar tables now in use. 



On an Instrument to illustrate Poinsot's Theory of Rotation, 

 By J. C. Maxwell. 



In studying the rotation of a solid body according to Poinsot's method, we have 

 to consider the successive positions of the instantaneous axis of rotation with refer- 

 ence both to directions fixed in space and axes assumed in the moving body. The 

 paths traced out by the pole of this axis on the invariable plane and on the central 

 ellipsoid form interesting subjects of mathematical investigation. But when we 

 attempt to follow with our eye the motion of a rotating body, we find it difficult to 

 determine through what point of the body the instantaneous axis passes at any 

 time,^and to determine its path must be still more diflScult. I have endeavoured to 

 render visible the path of the instantaneous axis, and to vary the circumstances of 

 motion, by means of a top of the same kind as that used by Mr. Elliot, to illustrate 

 precession*. The body of the instrument is a hollow cone of wood, rising from a 

 ring, 7 inches in diameter and 1 inch thick. An iron axis, 8 inches long, screws 

 into the vertex of the cone. The lower extremity has a point of hard steel, which 

 rests in an agate cup, and forms the support of the instrument. An iron nut, 

 three ounces in weight, is made to screw on the axis, and to be fixed at any point ; 

 and in the wooden ring are screwed four bolts, of three ounces, working horizontally, 

 and four bolts, of one ounce, working vertically. On the upper part of the axis is 

 placed a disc of card, on which are drawn four concentric rings. Each ring is 

 divided into four quadrants, which are coloured red, yellow, green, and blue. The 

 spaces between the rings are white. When the top is in motion, it is easy to see in 

 which quadrant the instantaneous axis is at any moment and the distance between 

 it and the axis of the 'instrument ; and we observe,^lst. That the instantaneous 

 axis travels in a closed curve, and returns to its original position in the body. 

 2ndly. That by working the vertical bolts, we can make the axis of the instrument 

 the centre of this closed curve. It will then be one of the principal axes of inertia. 

 Srdly. That, bj- working the nut on the axis, we can make the order of colours either 

 red, yellow, green, blue, or the reverse. When the order of colours is in the same 

 direction as the rotation, it indicates that the axis of the instrument is that of great- 

 est moment of ineirtia. 4thly. That if we screw the two pairs of opposite horizontal 

 bolts to different distances from the axis, the path of the instantaneous pole will no 

 longer be equidistant from the axis, but will describe an ellipse, whose longer axis is 

 in the direction of the mean axis of the instrument. 5thly. That if we now make one 

 of the two horizontal axes less and the other greater than the vertical axis, the instan- 



t 



* Transsactions of the Royal Scottish Society of Arts, 1855. 



