12 



BULLETIN 314, U. s. DEPARTMENT OF AGRICULTURE. 



a 



b~~^ 



a~~~- 



opens the clamp. By turning this button while the clamp is beino- 

 held open, it will lock and keep the clamp from closing until unlocked. 

 The device for measuring the distance penetrated by the needle 

 consists - of a rack, with a foot g. The movement of this rack turns 

 a pinion, to which is attached the hand which indicates on the dial 

 7b the vertical distance covered by the rack. One division of the dial 

 corresponds to a movement of 0.1 millimeter by the rack. The rack 

 may be raised or lowered by moving the counterweight i up or 

 down. The tin box containing the sample to be tested is marked Jc; 

 this is submerged in water contained in the glass cup in order to 



maintain a constant 

 temperature. 



This apparatus is 

 known as the Dow pen- 

 etration machine. An- 

 other type of machine 

 known as the New 

 York Testing Labora- 

 tory penetrometer, 

 based upon the same 

 general principle and 

 using the same stand- 

 ards, is at present em- 

 ployed by the Office of 

 Public Roads and Rural 

 Engineering. This 

 penetrometer is shown 

 in figure 7. Both ma- 

 chines give practically 

 the same results, if op- 

 erated under the same conditions, and it is therefore considered un- 

 necessary to include a description of the latter. 



A cup suitable for holding the box containing the test sample dur- 

 ing penetration is conveniently made from a glass crystallizing dish 

 10 centimeters in diameter, with straight sides about 6 centimeters 

 high. Three right triangles with right angle sides 1 and 5 centime- 

 ters, respectively, are cut from xg-inch sheet metal. Some solid bitu- 

 men is melted in the bottom of the dish forming a layer about f-inch 

 thick, into which the triangles are placed, resting on the side five cen- 

 timeters long. Their apexes should meet at the center, with their 

 short sides dividing the circumference of the dish into three equal arcs. 

 When the bitumen has hardened, the triangles give a firm support for 

 circular boxes, and the possibility of any rocking motion and conse- 

 quent faulty results is avoided. 



-^ 



Fig. 6. — Dow penetration machine. 



