156 REIBMNG AND SALINGEE. 



cases will secure good material, but often at the expense of rejecting 

 equally good or better material." 



Merz, Meyer, Schiffner, and others also insist that even when cement 

 is gauged to the proper plasticity there is a large personal error due to 

 the oj)erator himself. After a careful study of this personal error, we 

 have come to the conclusion that it is mainly due to the following five 

 causes : 



1. The manner of ajjplying tlie needles. 



2. The presence of small air bubbles near the surface of the pat. 



3. The difference in the amount of water brought to the surface in 

 patting the cement together and its presence there in a more or less 

 liquid layer. 



4. The dilBculty in judging the exact time when the needles cease 

 to make a "visible impression." 



5. The difference in plasticity. 



To overcome the first difficulty the pat should be made with a flat 

 (not rounded as specified) top as illustrated in fig. 1. The needle should 

 then be applied very gently and after the flat point rests upon the 

 surface of the pat the full weight of the needle should gradually be 

 applied. Failure to hold the needles in an exactly vertical position will 

 often cause the edges to indent where the flat point would not. 





To overcome the second, third and fifth difficulties, the cement is 

 gauged with the least amount of water which', after one minute's soaking 

 and four additional minutes of vigorous troweling will produce a paste 

 sufficiently stiff to retain its shape, and yet so plastic that the initial 

 needle will sink almost to the glass plate when applied directly after 

 forming the pat. A ball of this paste when dropped from a height of 

 70 centimeters will fiatten very slightly and will not crack. A lump 

 dropped from the point of the trowel will leave the surface of the latter 

 comparatively clean. In forming the pat the cement should be thoroughly 

 patted together with the flat of the trowel. This eliminates the air 

 bubbles near the surface and also brings the excess of water to it. In 

 forming the flat top, the hyperaqueous cement should be wiped oil as 

 much as possible with the edge of the trowel, and the surface left smooth 

 and firm. 



Difficulty number five is especiallj' marked in slow-setting cements, 

 as sometimes a slight indentation will persist for hours and in the judg- 

 ment of an individual operator, may even not be fixed within thirtj' to 



/ 



