T20 JSTEW YOEK STATE MUSEUM 



placed on top of the soft cement projecting above tlie mold." 

 The loading block is shaped to the mold with -gV of an inch clear- 

 ance and is to be placed on the cement sjTiimetrically ; after load- 

 ing, the surplus cement is to be cut off with a trowel or a knife, 

 and the briquet smoothed level with the top of the mold. " With 

 the exercise of the greatest care, the handmade briquets do not 

 compare with those made with the machine.'' 



Another advantage of machines is the rapidity with which the 

 briquets can be made, and an additional advantage of this is that 

 a lot of material can be mixed up at once. 



In making briquets by hand enough material is usually mixed 

 to make four or five briquets at once, and this is necessary if the 

 material is at all quick setting. 



• All tensile test briquets should be kept in a moist atmosphere 

 for 24 hours, and then kept the remainder of the period in water. 

 It is important that the water used in mixing and also the bath in 

 which the briquets are immersed should be kept at a constant tem- 

 perature, so that uniform results may be obtained. Thus it has 

 been found that in Portland cement the time of setting is shortened 

 by increasing the temperature of the mixing water, while the 

 strength attained in a given time may be greatly increased by rais- 

 ing the temperature of the bath from 40° to 80°\ In case of 

 normal mortar, IC: 3S, this increase at two months was from 100 

 to 230 pounds per square inch. 



Briquet machines. The object of these is to bring about uni- 

 formity of pressure in the molding of the briquets. A number of 

 such machines have been devised but comparatively few of them 

 are in use. The Bohme hammer is a machine much used in Ger- 

 many for this purpose (pi. 23). According to M. Gary^ it con- 

 sists of a tilt hammer with automatic action. The hammer is 

 driven by a cam wheel of 10 cams actuated by simple gearing, and 

 the wrought iron handle of the hammer is let into the crosshead 



2 Johnson. Materials of construction, p. 408. 

 Trans. Am. soc. civ. eng. 30: 24. 



