used, and the ink being the complement in color of the color of the screen used in taking the photograph. The process is again repeated, using the third halftone block. To secure good results, it is necessary that perfect registration be obtained, that is, the second and third printings must be exactly superposed on the first. Fig. 40 shows the various steps in the printing, the little squares showing the color (or colors) of the ink used. A is the first printing, C the second, and E the third and final printing. It will be instructive to examine A, B, and D under a magnifying glass, which will bring out more clearly faint outlines that appear clearly in C and E.

167. Paper is colored for two reasons; first to produce an approximate white, and second to produce a definite color, as green or pink. Paper pulp usually has a yellowish tint, reflecting a preponderance of yellow rays. To correct this a proper mixture of red and blue coloring matter is added, so that the light reflected by it will be complementary to the yellow and so compensate or neutralize it. Other colors can likewise be compensated, but the mixing of pigments or dyestuffs always is a step toward darkness.

Coloring paper to a definite shade is done by applying the principles explained in Art. 166. It makes no difference whether the coloring matter is a pigment that mixes with the fibres, or a dyestuff that stains them; the color of the paper is determined by the rays left to be reflected after part have been absorbed by the colored fibres.

ELEMENTS OF PHYSICS

(PART 2)

EXAMINATION QUESTIONS

(1) Steam is cut off in an engine cylinder at 3ths stroke and expands to the end of the stroke. Assuming that the fall in pressure follows Boyle's law for increase in volume (which is approximately true), what will be the pressure at the end of the stroke, if the pressure at cut off is 126 lb. per sq. in. gauge? Ans. 38+ lb. per sq. in. gauge.

(2) If 4.6 cu. ft. of air at 96°F. expand at constant pressure until the temperature becomes 53°F., what will be the volume? Ans. 4.244-cu. ft.

(3) A vessel holding 1.58 cu. ft. is filled with air at a pressure of 14.65 lb. per sq. in. abs.; if the temperature of the air is 70°F. and it is heated to 700°F., what will be its pressure?

Ans. 32.08 lb. per sq. in. abs. (4) If 2.66 cu. ft. of air at 62°F. and a pressure of 1 atmosphere, is compressed to .52 cu. ft., what will be the tension when the air has a temperature of 112°F.?

Ans. 67.71 lb. per sq. in. gauge.

(5) Taking the specific gravity of nitrogen as .971, what is the weight of 875 cu. ft. when the tension is 20 lb. per sq. in. abs. and the temperature is 80°F.? Ans. 85.186 lb.

(6) Referring to the last example, what is the weight of 1 cu. ft. of nitrogen at 32°F. and a tension of 1 atmosphere?

Ans. .078517 lb.

(7) Referring to Questions 5 and 6, if 3.8 cu. ft. of nitrogen at 60°F. and a tension of 1 atmosphere is heated at constant volume to 1000°F., (a) how many B.t.u. must be expended? (b) how many foot-pounds of work are equivalent to this? (c) what will be the tension?

Ans.

(a) 46.7 B.t.u.

(b) 36,333 ft.-lb.

(c) 41.3-lb. per sq. in. abs.