RURAL ENGINEERING. 191 



for grades exceeding 20 per cent about 100 to 150 yds. apart. The resting places 

 sliould be about 30 ft. in length and should not exceed a 5 per cent grade. The 

 best grade between any two points is upon a line having the same percentage 

 from beginning to end, and the avoidance of reversed grades, if possible, is rec- 

 ommended. 



Under construction it is stated that the width of clearing should be sufficient 

 for the easy passage of loaded pack animals. A maximum width of trail, on 

 solid foundation, of 48 in. is considered sufficient, while 15 in. is the minimum 

 set for class A trails. The tread should have a slope toward the hill of about 1 

 in. per foot. In grading, the cut bank should be sloped according to the angle 

 of repose of the material sufficiently to prevent sliding from the upper side. If 

 a switchback is found necessary it should be made so that a horse can walk 

 around the turn. Turns should be made level and with a minimum radius of 

 4 ft., and the grade of the trail approaching and leaving the turn should not 

 exceed 5 per cent for a distance of a few yards. 



Other general information regarding construction is given, including drain- 

 age, corduroy construction, and slide and solid rock work. 



With reference to bridges, it is stated that the factors affecting the selection 

 of a bridge site are, in order of their importance, shortness of span, favorable 

 banks, minimum range of high water, straight and unobstructed channel above 

 and below, and accessibility. The ideal site is one where the stream is narrow 

 and straight and affords free and unobstructed flow, even in flood stage, and the 

 banks are high and of solid material. Specimen designs for types of bridges, 

 with dimensions of members for different spans, are given. It is stated that 

 for spans less than 20 ft. a simple nontruss stringer bridge will adequately meet 

 the needs, while for spans between 20 and 36 ft. the king truss design will ordi- 

 narily be most suitable, and the queen truss for spans between 36 and 60 feet. 

 Cable suspension bridges are considered to be especially applicable to spans of 

 from 75 to 200 ft., while trestle bridges are used only to cross deep canyons or 

 draws with a small stream flow. 



" On account of the variability in the strength of gi-een timber, bridges of such 

 material are commonly built with a large factor of safety." Under existing 

 conditions of trail work on the forests, it is stated that stringers less than 12 in. 

 in diameter are not generally used in spans of 8 ft. or over. 



Tables are given showing the minimum safe dimensions for bridge members. 

 The bridge plans shown are designed for a total load, live and dead, of 125 lbs. 

 to the square foot, and when good structural timber is used afford a factor of 

 safety of 6. In this connection it is stated that snow loads on forest bridges 

 usually exceed any live loads to which such structures may be subjected. 



Other general information is given regarding bridge construction and a final 

 section is included on administration. A section enumerating the tools necessary 

 in trail and bridge work is appended. 



Telephone construction and maintenance on the National Forests {U. S. 

 Dept. Ayr., Forest Serv., Telephone Construction and Maintenance on the Na- 

 tional Forests, 1915, pp. 83, pis. 2, figs. ^3). — This pamphlet gives detailed in- 

 structions, with illustrations, as to the construction and maintenance of tele- 

 phones in National Forests. A form of contract between a commercial tele- 

 phone company and the Secretary of Agriculture for telephone service in con- 

 nection with the National Forests is appended. 



A small aero-electric plant, E. H. Williamson, Jb. (Sci. Amer., 113 {1915), 

 No. 10, pp. 200, 201, figs. 6). — Directions for constructing a small wind power 

 plant for the generation of electrical current are given, with illustrations. 



The generation of hydrocyanic acid gas in fumigation by portable ma- 

 chines, H. D. Young {California Sta. Circ. 139 {1915), pp. 8, figs. 5).— This cir- 



