go WILLIAM H. HOBBS 



compose the sides of the triangle are double and provided with 

 clamp screws to permit of fastening the rubber sheet at its margins. 

 When in usej the frame is placed upon a ring tripod, and since 

 the frame is supplied with a window in its bottom, cold masses of 

 metal of any desired shape may be introduced below and brought 

 in contact with the under surface of the rubber sheet in order to 

 induce local premature rigifaction of the cooling layer of balsam 

 resting upon the sheet. 



The principal difficulties encountered in carrying out the tests 

 are due to the necessity of evaporating the balsam so that it will 

 rapidly rigify in cooling and still be sufficiently fluid when poured 

 upon the sheet to spread out into a thin layer. The evaporation 

 can be accomplished upon a water bath, but a slightly higher 

 temperature given it just before pouring is desirable. If the 

 temperature is carried too high the surface of the rubber may be 

 seriously affected. 



The series of concentric arcs which appear in the balsam layer 

 represented in Fig. 8 are due to the local rigifaction of a circular 

 area near one side of the triangle but not centered upon a bisectrix, 

 with the principal contraction of the sheet from the two sides 

 opposite the convexity of the arc. 



