On the Rotation of Hollow Spheres of Metal by Heat. 95 



between their edges is 1^ inch, and they each taper from a sharp 

 edge at the top to about fths of an inch thick at the bottom. 

 The edges of the rails are turned perfectly true and smooth*, and 

 in some instances have been coated thickly with silver by electro- 

 deposition in order to protect the copper at those parts from 

 oxidation when subjected to a high temperature, but with unsa- 

 tisfactory results ; the greater expansibility of the silver caused 

 it to separate from the ring in the form of corrugations and 

 large blisters. B B B is a horizontal brass triangle with three 

 brass pins (two only, C, C, being shown in the cut) projecting 

 from its upper surface to support the nug. The triangle is sup- 

 ported by three long brass screws, D, D, D, provided with bone 

 feet to obstruct the transmission of heat to the table, and with 

 set-screws, E, E, to hold them steady when adjusted. F F, fig. 2, 



Fig. 2. (Scale ith.) 



is a flat circular groove, half an inch wide and ^\jth of an inch 

 deep, turned in the base of the outer rail to receive the ends of 

 the pins C, C. There are three longer pins fixed in the triangle 

 (of which only two, G, G, are shown), to assist as guides in placing 

 the red-hot ring upon the triangle; both these pins and the 

 pins C, C must be so fixed as to allow for the expansion of the 

 copper ring, the amount of which is nearly f ths of an inch in 

 its diameter at a full red heat. H is a spherical shell of German 

 silver, 4 inches in diameter, as thin, as perfect in form, and as 

 uniform in thickness as it can possibly be made. 



As the phsenomenon of rotation in this case is much more 

 delicate than in the other by electricity, already referred to, and 

 the success of the experiment depends greatly upon having suit- 

 able balls, and these are difficult to obtain, I give the following 

 particulars respecting the mode of forming them. A stout plate 

 of brass is turned into a ring whose inner diameter is the same 

 as that of the required ball ; it is then divided into two equal 

 parts, and one of these is retained to serve as a pattern or tem- 

 plate by which to form the hemispheres composing the ball. A 



* The rinp; should be previously heated to bright redness and slowly 

 cooled, otherwise the heat to which it is subjected in the experiments wdl 

 alter its form. 



