722 NEW YOKK STATE MUSEUM 



Cement-testing machines. A number of different machines 

 have been devised. The two generally used in this country are 

 Fairbanks's and Richie's. 



In the Riehle machine (pi. 24) the strain is applied to the 

 specimen by means of a screw attached to the lower clip. The 

 upper clip is attached to a graduated steel bar on which there slides 

 a weight. This bar is kept balanced during the test by moving the 

 weight along it with the aid of the upper wheel. Both wheels 

 must be operated at the same time, and the end of the upper beam 

 or indicator kept as nearly as possible in the middle of the open- 

 ing in which it moves. The amount of strain on the specimen is 

 shown by the position of the sliding weight. 



The Fairbanks machine is of more compact form, and its con- 

 struction is best understood by reference to the figure. After 

 putting the briquet into the clips the levers are balanced, and the 

 hopper filled with shot. This is allowed to run out into the 

 bucket till the briquet breaks, when the stream of shot is stopped 

 automatically (pi. 25). 



Clips. Several forms of clips are made. The early ones had 

 rather sharp edges which came in direct contact with the briquet; 

 but this has been found objectionable, partly from the fact that 

 the briquets were not always of just the right form to insure a 

 perfect bearing. The result of this Avas that a false strain was 

 often brought on the briquet, causing it to break at a lower point 

 than it really should, and also at some other point than its mini- 

 mum section. This trouble has been overcome in a measure by 

 introducing cushions between the metal and the briquet, or even 

 supplying the edges of the briquet with rubber rolls. 



Johnson gives the essential features of the clips as follows. 



1 They must grasp the briquet by a hard cushion bearing on 

 four symmetric fiat surfaces. 



2 They must be freely suspended from a pivot bearing, so as 

 to turn without friction while under stress. 



