W. E. Dalby 



41 



forms of the machine. In other forms the three points A, B,C are in 

 the lower surface of the timber sample and the point Z is in the upper 

 surface so that the plane of reference is above the plane of the machine 

 by approximately the thickness of the sample. 



Fig. 2 



It is only when the sample is clamped at three points that a true 

 record of the surface can be made. The act of clamping at more 

 than three points distorts the timber so that the family of profile 

 curves correspond to a surface slightly strained, and the surface 

 will therefore change in shape immediately the sample is removed from 

 the machine. When however it is clamped at three points only the 



PIA l^e- of^ MAC.t-iiMic 



Fig 3 



sample is quite unstrained, and moreover it can be removed from the 

 machine and can be replaced in it again in the same position relatively 

 to the frame, providing always that it is clamped at the same three 

 points. The initial clamping points should therefore be carefully 

 marked on the sample by ringing them round or by any other con- 

 venient method. 



3—5 



