12 BULLETIN 314, U. S. DEPARTMENT OF AGRICULTURE. 
ee 
opens the clamp. By turning this button while the damp is being 
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 
@ pinion, to which is attached the hand which indicates on the dial 
h 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 k; 
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 Roadsand 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. ey | 
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 34-inch sheet metal. Some solid bitu- 
men is melted in the bottom of the dish forming a layer about $-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. . ) 
Fie. 6.—Dow penetration machine. 
