EQUIPMENT 



409 



satisfactory. It was cumbersome, difficult to read, and only very approxi- 

 mately accurate. To replace this, a "slope-card" was designed by adding 

 somewhat to the arm protractor and goniometer patented by the late 

 Professor Penfield and used for measuring crystals (see figure 3). It con- 

 sists of a rectangular, heavy, stiff cardboard, 6 31^4 inches wide, con- 

 taining a 180-degree scale, numbered from both ends, with additional 

 centimeter and inch scales for convenience. A movable celluloid arm. A, 

 pivots at the middle of the diameter of the semicircle C, equivalent to 

 the index point of any protractor. A pendulum, D, swings freely from 



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Figure 3. — "Slope Card" to replace slope Board 



Adapted from arm goniometer designed by S. L, Penfield. A is movable celluloid arm 

 pivoting at C, along top of which sight is taken. Reading is taken where line E inter- 

 sects graduated scale. B is a double protractor scale graduated to 180 degrees, with 

 zero at the top on both sides. A freely swinging pendulum, D, pivots at C. 



the same index point. It is merely a piece of stiff steel wire with the 

 index end pointed and the other end bent into a loop which allows it to be 

 suspended as a pendulum in the hole at C. 



In use the card is held with its longer edge firmly on the plane-table, 

 with the round part of the semicircle downward. The table is adjusted 

 until the swinging pendulum points to 90 degrees, or the center of the 

 protractor, thereby indicating that it is level. The object is then sighted 

 along the blackened edge of the movable celluloid arm A, shown in figure 

 3, and the angle of slope read directly by means of the scratched line E, 

 on the under side of the arm. Positive angles are recorded on one side 



