1916] RURAL ENGINEERING. 87 



effect. The stresses in this slab will be a maximum under the load and will 

 decrease in each direction from it. The * effective width ' of a slab is that 

 width used in designing over which a single concentrated load may be consid- 

 ered as uniformly distributed on a line down the middle of the slab parallel 

 to the supports. 



"The tests of slabs seem to warrant the following conclusions: (1) The 

 ' effective width ' is affected very little by the percentage of transverse rein- 

 forcement (parallel to supports). (2) The 'effective width' decreases some- 

 what as the load increases. (3) The 'effective width' in percentage of the 

 span decreases as the span increases. (4) The following formula will give a 

 safe value of ' effective width ' where the total width of slab is greater than 

 li S+4 ft: 6=0.6 S+1.7 ft., where e=effective width in feet and /S'=the span 

 in feet." 



Charts for estimating the strength of bolts, W. F. Fishek {Power, 43 

 (1916), No. 2, pp. JfZ, 43, figs. 2). — Two charts are presented, showing the 

 strength of threaded bolts from 0.25 to 2.5 in. in diameter. These were pre- 

 pared from the formula L—AXf, in which L=the load on the bolt in pounds 

 in tension, A=the area in square inches of the bolt at the root of the thread, 

 and /=the unit fiber stress on the bolt material at the root of the thread in 

 pounds per square inch. 



Explosion period in gas engine, R. S. King {Poioer, 43 (1916), No. 2, pp. 

 48, 49, fig. 1). — The results of experiments on a 6-horsepower, 4-stroke-cycle, 

 hit-and-miss governed gas engine with make-and-break ignition are reported, 

 which indicate that the rapidity of explosion is greatest with the best mixtures 

 and increases with the compression. 



Gasoline farm tractors, P. S. Rose {Engin. Mag., 40 (1915), No. 5, pp. 750- 

 152, figs. 6). — This is a review of the development during the past few years of 

 internal combustion machines for heavy haulage about the farm. 



Demonstrations of motor plows and tractors (Jour. Bd. Agr. [London], 22 

 (1915), No. 8, pp. 760-766). — A brief description of different motor plowing out- 

 fits and a review of results obtained in demonstrations at different localities in 

 England are given. 



Demonstration of mechanical cultivation and of agricultural motors, 

 Parma, 1913, M. Castelli and G. D. Mayer (Concorsi di Aratura Meccanica 

 e di Motori Agricoli. Milan: Cattedra Amhulante d'Agricoltura di Parma, 1913, 

 pp. 234, figs. 57). — The first part of this report describes and illustrates a num- 

 ber of mechanical cultivating outfits of both American and European make, in- 

 cluding internal-combustion motor plows and cultivators, internal-combustion 

 and steam tractors, and cable plowing outfits, and presents and discusses the 

 results of tests of each on a soil of widely varying texture. The second part 

 describes and illustrates a number of stationary and portable internal combus- 

 tion farm engines, mainly of European make, and reports and discusses tests of 

 each. In both sets of tests internal-combustion motors were included, operat- 

 ing on high- and low-grade fuels. 



The mechanical cultivation of the soil, P. Mathis (Agr. Colon. [Italy], 9 

 (1915), Nos. 3, pp. 129-147; 4-5, pp. 227-249, figs. 5; 6, pp. 345-358, figs. 8; 7, 

 pp. 403-419, figs. 5). — This article points out the importance of mechanical cul- 

 tivation in the agriculture of colonial Italy, describes steam, electrical, and 

 internal-combustion power in their application to mechanical cultivation, and 

 describes and discusses a large number of mechanical plowing outfits, including 

 tractors, motor plows, and cable outfits of both American and foreign make. 



The relation of mechanical cultivation to intensive agriculture, A. Datjtey 

 (In Primera Semana Social Agricola. Santiago de Chile: Universidad Catdlica 

 43795°— No. 1—16 7 



