3g TRANSPORTATION 0\ LAND BY VEH1CLES:-THE ROADS 



(b) Spirit level instruments. 



Spirit level instruments seem to be more exact but are no more exact than plumb bob instruments. 

 Anv spirit level used by the mason, the carpenter, or the plumber can be used in connection 

 with a target graded in inches and feet, to ascertain or to lay out the grade of a road. 



1. The small pocket instruments, seemingly handy, are apt to be very inexact, particularly so when 

 they have a vernier outfit. The so-called "altimeters" consist of a short telescope and of a dial 

 topped by a spirit level. With the help of a mirror placed inside of the telescope, and through 

 a hole above it, the eye controls the bubble in the spirit level while sighting simultaneously 

 through the telescope. The readings are taken from the dial of an arc graded in degrees. 



2. The Staiidinger has a long open sight line, is securely fastened to a tripod, and works with 

 the exactness of a highgrade transit. The horizon is established by a large spirit level, and 

 the objective slides over units of gradation forming the 100th part of the distance between ocular 

 and objective. 



II. Instruments for measuring horizontal angles. A common compass, set on a Jessup staff, is the 

 best and handiest instrument for forest use. A small plane table can be used to excellent advantage, also, 

 in the construction of roads, road curves, and so on. 



For description of these instruments compare Gary's "Handbook for Northern Woodsmen," Harvard, 

 1909, page 91 and 95. 



III. Instruments for measuring both vertical and horizontal angles. An ordinary surveyor's transit is 

 best adapted to this purpose. All transits have, unfortunately, telescopic sights and all of them require a 

 substantial tripod. .Ml of them, in the hands of a man who does not handle a transit continuously, are apt 

 to result in misuse and mistakes. Compare Gary, Handbook, page 69. 



Barometrical measurements are valuable in preliminary surveys. 



(G) FRICTIONAL RESISTANCE ON ROADS. Friction is the force of resistance to the passage of 

 one substance over another. 



Sliding friction is much greater than rolling friction. 



Friction is due partly to adhesion, and partly to the roughness of the surfaces in contact. The amount 

 of friction is proportioned to the pressure exerted perpendicularly on the lower surface. Similar substances 

 excite a greater degree of friction than dissimilar substances. 



Velocity augments friction slightly. 



The amount of friction is independent of the extent of the surfaces in contact. Thus the friction 

 of a body sliding upon another will be the same whether the body moves upon face or upon edge. 



Waggon wheels and sled runners 4 inches wide create no more friction than those 2 inches wide. 



The "angle of repose" is defined as the greatest angle of obliquity between two planes consistent 

 with stability. 



Frictional resistance is that portion of the pressure of a moving body which must be exerted by a 

 counter force to overcome the resistance to moving. 



Under "coefficient of friction," f, is understood the ratio existing between frictional resistance, R, and 

 pressure, P, or, which is the same, the frictional resistance per one lb. of pressing weight. 



t = -^ 



P 



(f = coefficient of friction ; R ^ frictional resistance ; P = pressure of body on incline.) 



"Coefficient of friction" is that factor by which pressure must be multiplied in order to obtain a 

 counter force just sufficient to overcome the inclination of a body to slide on the "angle of repose." 

 The tangent of the angle of repose equals the coefficient of friction. 



^ f 



