486 EXPERIJVIEXT STATION RECORD. [Vol.41 



It is concluded that "(1) crushed stone remains intact and resists impact 

 better than gravel. (2) Large size stone and gravel are more durable than the 

 small sizes. (3) The re.sistance to impact depends upon the quality of both 

 the mortar and the coarse aggregate and, although a good matrix has a ten- 

 dency to support a weak coarse aggregate, the use of a good quality of the 

 latter will make a more durable pavement. (4) A coarse-grained sand mortar 

 i-esists impact better than a mortar made with finer-grained sand. (5) No 

 relation has been observed between compressive strength and resistance to 

 impact. . . . An increase of cement content increases resistance to wear, but 

 with a proper selection of aggi-egates of excellent quality a 1:2:4 mix can be 

 made which will offer a better resistance to impact than a poorer quality of 

 aggregate mixed 1 : 1..5 : 3." 



Effect of tractive resistance in terms of gasoline consumption (Engin. and 

 Contract., 51 (1919), No. 19, p. 482, figs. 3). — Three charts are given showing the 

 relation between tractive resistance of road surface, annual cost of roadbed, 

 and grade on gasoline consumption for motor veliicles. 



An economic study of the gas tractor in Pennsylvania, D. S. Fox (Penn- 

 sylvania Sta. Bui. 158 (1919), pp. 3-20, figs. 5).— This bulletin reports data 

 derived from detailed records secured from 54 farms in central and southeastern 

 Pennsylvania, on which gas tractors were operated during the year ended in 

 the spring of 1918. Tractors Avere used chiefly on the larger farms, 123 acres 

 of crops per farm being the average. The average purchase price was $959, 

 and the average estimated life of the tractor was 8.1 years. Kerosene at 12 

 cts. per gallon was a cheaper fuel than gasoline at 25 cts. The cost per 10- 

 hour day was $2.06 for kerosene and $3.32 for gasoline, and per horsepower 

 hour 2.2 and 3.1 cts., respectively. 



The average tractor did 50.6 days' work. There was an average of 12.8 days 

 of plowing, 9.3 days of harrowing, and 19.6 days of belt work. The average 

 tractor used a little over 1 gal. of cylinder oil worth 45 cts. for each 10-hour 

 day. The average cost of operation including operator's labor was $622.12 for 

 50.6 days, or an average of $12..30 per day. The cost excluding the operator's 

 labor was $495.62, or $9.80 per day. The size of farm was increased in only 

 two cases, and more intensive crops were grown on only 7 farms of the 52 that 

 were operated before the purchase of the tractor. 



The most important factor affecting the cost of operation was days of work 

 done per tractor. Tractors working 30 days or less, or an average of 24.3 days, 

 cost $19.97 per day, while tractors working over 70 days, or an average of 84.9 

 days, cost only $9.85 per day. In the second case the overhead cost was dis- 

 tributed over more days of work. 



The average tractor cost of plowing was $2.33 per acre. The plow cost was 

 71 cts. per acre, making a total of $3.04. The tractor cost for harrowing and 

 disking was 69 cts. per acre for once over the ground, and excluding the cost 

 of tillage implements used. 



The farmer's son was the most efficient operator. The average cost per day 

 was as follows: Son $10.75, owner $12.16, and hired man $12.82. An average 

 of 1.8 horses were disposed of on 48 farms. The cost of operation, less the 

 operating labor, was $275.34 for each horse displaced. 



There was a change in layout on only two farms. The fields on most farms, 

 however, should be rearranged for oflicient tractor operation. 



Tractoj test on the university farm, .7. A. Howenstine (Agr. Student, 2^ 

 (1918), No. 9, pp. 542, 543, fig. 1).—A test was conducted on a 12-20 h. p. tractor 

 plowing 8 in. deep on heavy black clay soil at the Ohio State University. Kero- 

 sene was used for fuel, gasoline being required only for starting and priming. 

 A three-bottom gang plow and a harrow were used. The average speed was 



