Month	Thousands Of Service Hours
1921-07-01	309538.
1921-08-01	335807.
1921-09-01	328288.
1921-10-01	350976.
1921-11-01	328012.
1921-12-01	310547.
1922-01-01	292919.
1922-02-01	272065.
1922-03-01	314461.
1922-04-01	294002.
1922-05-01	319069.
1922-06-01	328681.
1922-07-01	293729.
1922-08-01	348284.
1922-09-01	362922.
1922-10-01	391831.
1922-11-01	375429.
1922-12-01	368692.
1923-01-01	375162.
1923-02-01	336780.
1923-03-01	386624.
1923-04-01	370254.
1923-05-01	395264.
1923-06-01	393970.
1923-07-01	395042.
1923-08-01	411540.
1923-09-01	378665.
1923-10-01	409504.
1923-11-01	368713.
1923-12-01	341646.
1924-01-01	350245.
1924-02-01	330780.
1924-03-01	352571.
1924-04-01	348309.
1924-05-01	353867.
1924-06-01	334572.
1924-07-01	346254.
1924-08-01	350013.
1924-09-01	349224.
1924-10-01	376604.
1924-11-01	337567.
1924-12-01	343423.
1925-01-01	347615.
1925-02-01	311835.
1925-03-01	341865.
1925-04-01	339103.
1925-05-01	343365.
1925-06-01	349106.
1925-07-01	356997.
1925-08-01	356999.
1925-09-01	350232.
1925-10-01	373996.
1925-11-01	342086.
1925-12-01	346976.
1926-01-01	337030.
1926-02-01	315168.
1926-03-01	357259.
1926-04-01	349131.
1926-05-01	354561.
1926-06-01	361489.
1926-07-01	369022.
1926-08-01	371342.
1926-09-01	363104.
1926-10-01	377978.
1926-11-01	356760.
1926-12-01	353575.
1927-01-01	337980.
1927-02-01	308187.
1927-03-01	351255.
1927-04-01	343597.
1927-05-01	352007.
1927-06-01	358074.
1927-07-01	350777.
1927-08-01	363711.
1927-09-01	344736.
1927-10-01	356202.
1927-11-01	328921.
1927-12-01	319877.
1928-01-01	310953.
1928-02-01	294389.
1928-03-01	325181.
1928-04-01	313652.
1928-05-01	338264.
1928-06-01	335483.
1928-07-01	332585.
1928-08-01	349127.
1928-09-01	324244.
1928-10-01	354518.
1928-11-01	320800.
1928-12-01	305625.
1929-01-01	315240.
1929-02-01	287475.
1929-03-01	321954.
1929-04-01	326000.
1929-05-01	341684.
1929-06-01	332940.
1929-07-01	347473.
1929-08-01	355769.
1929-09-01	330463.
1929-10-01	358144.
1929-11-01	317266.
1929-12-01	303627.
1930-01-01	303491.
1930-02-01	269841.
1930-03-01	297064.
1930-04-01	298952.
1930-05-01	306026.
1930-06-01	286647.
1930-07-01	285928.
1930-08-01	281501.
1930-09-01	268621.
1930-10-01	278234.
1930-11-01	242074.
1930-12-01	243573.
1931-01-01	240460.
1931-02-01	214549.
1931-03-01	239619.
1931-04-01	238730.
1931-05-01	237741.
1931-06-01	234276.
1931-07-01	234281.
1931-08-01	225761.
1931-09-01	213979.
1931-10-01	217125.
1931-11-01	189475.
1931-12-01	189763.
1932-01-01	182497.
1932-02-01	175202.
1932-03-01	191796.
1932-04-01	180774.
1932-05-01	179096.
1932-06-01	172381.
1932-07-01	162711.
1932-08-01	165450.
1932-09-01	161131.
1932-10-01	171454.
1932-11-01	161670.
1932-12-01	160793.
1933-01-01	152231.
1933-02-01	139232.
1933-03-01	149980.
1933-04-01	144232.
1933-05-01	156219.
1933-06-01	159183.
1933-07-01	165937.
1933-08-01	178130.
1933-09-01	171519.
1933-10-01	177263.
1933-11-01	162642.
1933-12-01	158145.
1934-01-01	164431.
1934-02-01	153775.
1934-03-01	176916.
1934-04-01	169459.
1934-05-01	182530.
1934-06-01	181736.
1934-07-01	178597.
1934-08-01	182371.
1934-09-01	167890.
1934-10-01	180946.
1934-11-01	162376.
1934-12-01	160828.
1935-01-01	170962.
1935-02-01	154191.
1935-03-01	170086.
1935-04-01	167183.
1935-05-01	174472.
1935-06-01	171584.
1935-07-01	177531.
1935-08-01	179423.
1935-09-01	169398.
1935-10-01	186886.
1935-11-01	169486.
1935-12-01	170927.
1936-01-01	180861.
1936-02-01	183398.
1936-03-01	188481.
1936-04-01	188648.
1936-05-01	190070.
1936-06-01	193329.
1936-07-01	200052.
1936-08-01	198736.
1936-09-01	198372.
1936-10-01	211777.
1936-11-01	192158.
1936-12-01	201178.
1937-01-01	196164.
1937-02-01	186726.
1937-03-01	211948.
1937-04-01	208608.
1937-05-01	211726.
1937-06-01	216817.
1937-07-01	218606.
1937-08-01	214580.
1937-09-01	203682.
1937-10-01	205818.
1937-11-01	184853.
1937-12-01	179328.
1938-01-01	167502.
1938-02-01	150797.
1938-03-01	169830.
1938-04-01	158478.
1938-05-01	157443.
1938-06-01	161308.
1938-07-01	162578.
1938-08-01	173633.
1938-09-01	171288.
1938-10-01	180429.
1938-11-01	170024.
1938-12-01	170726.
1939-01-01	167647.
1939-02-01	155288.
1939-03-01	177901.
1939-04-01	165193.
1939-05-01	175803.
1939-06-01	180939.
1939-07-01	179094.
1939-08-01	189008.
1939-09-01	185208.
1939-10-01	201294.
1939-11-01	188135.
1939-12-01	180939.
1940-01-01	186767.
1940-02-01	171331.
1940-03-01	180381.
1940-04-01	179107.
1940-05-01	187240.
1940-06-01	185127.
1940-07-01	196333.
1940-08-01	202361.
1940-09-01	190769.
1940-10-01	208026.
1940-11-01	189277.
1940-12-01	189433.
1941-01-01	194228.
1941-02-01	177461.
1941-03-01	202341.
1941-04-01	203080.
1941-05-01	218550.
1941-06-01	220434.
1941-07-01	233565.
1941-08-01	237017.
1941-09-01	231873.
1941-10-01	247137.
1941-11-01	222928.
1941-12-01	232099.
1942-01-01	231251.
1942-02-01	208935.
1942-03-01	237259.
1942-04-01	241688.
1942-05-01	247524.
1942-06-01	252943.
1942-07-01	261215.
1942-08-01	267397.
1942-09-01	262207.
1942-10-01	279252.
1942-11-01	259133.
1942-12-01	273748.
1943-01-01	270158.
1943-02-01	250690.
1943-03-01	284873.
1943-04-01	276156.
1943-05-01	279830.
1943-06-01	284871.
1943-07-01	291091.
1943-08-01	294559.