100 Mr. G. Gore on the Rotation of 



redness before placing each ball upon it. Ball M started at once 

 all round in a left-handed direction, and made 5| revolutions in 

 half a minute ; it then burst by expanded air, but continued its 

 motion and made 4| revolutions in the second half minute. 

 Ball N started with decidedly less vigour than ball M, and made 

 4 revolutions in a left-handed direction in half a minute. Ball 

 started with much greater energy than ball N, went at once 

 completely round, and made 6| revolutions in a left-handed 

 direction in half a minute. Ball P started with greater force 

 than ball 0, and made 8i revolutions in a left-handed direction 

 in half a minute ; owing to some defect either in its form or 

 distribution of weight, it made one long vibration before starting 

 on its journey. N.B. The ring was reheated for each ball. No 

 impetus was given by the hand to either of the balls, they were 

 simply placed upon the rails. The apparatus was also protected 

 from currents of air. 



As these experiments were only made with a view of obtaining 

 complete revolving motion, they were not, except in " No. 6," 

 conducted under such conditions of equal temperature, freedom 

 from currents of air, &c., as to be strictly comparative. •' 



The following appears to me to be an explanation of this 

 phsenomenon of motion : — Immediately the cold ball is placed 

 upon the heated rails, two small protuberances (produced by the 

 heat) emerge from its surface at the points of contact, whilst 

 two corresponding hollows are produced in the rails at the same 

 points by contact of the cold ball, but in consequence of the 

 inferior heat-conductivity of German silver, and the thinness of 

 the spherical shell, the heat is prevented from quickly spread- 

 ing, and the maocimum of local expansion is produced in the 

 ball, whilst at the same time, from the superior heat-conduc- 

 tivity of copper and the massiveness and form of the ring, 

 the minimum of local contraction is produced in the rails, and 

 thus the protuberances in the ball are greater in amount than 

 the hollows in the rails, and the ball is at once placed in a state 

 of unstable mechanical equilibrium, somewhat like an egg placed 

 upon its end in a watch-glass, and is caused to move either in 

 one direction or the other by the slightest disturbing influence, 

 such as a breath of air, a minute inequality in the original form 

 or distribution of weight of the ball, &c. 



To account for the continuance of the motion, I suppose that 

 a minute period of time is occupied by the heat in producing 

 the expansion, and that during this period the ball has by its 

 momentum moved forwards a minute distance; and thus the 

 ever-recurring points of maximum expansion are always kept a 

 little behind the line of the centre of gravity of the ball, and 

 act as a propelling power, pushing it forwards by gentle leverage. 



