76 HIGHWAY .CONSTRUCTION 



the sum of the widths of the two sidewalks at the corner, and dividing 

 the whole by two. This gives an elevation equal to the average eleva- 

 tion of the curbs at the corners, plus an average rise of two and one- 

 half per cent across the width of the sidewalk. 



Accommodation summits have to be introduced between street 

 intersections first, in hilly localities, to avoid excessive excavation; 

 and second, when the intersecting streets are level or nearly so, for the 

 purpose of obtaining the fall necessary for surface drainage. 



The elevation and location of these summits may be calculated 

 as follows: Let A be the elevation of the highest corner; B, the eleva- 

 tion of the lowest corner; D, the distance from corner to corner; 

 and R, the rate of the accommodation grade. The elevation of the 

 summit is equal to 



D-R + A + B. 

 2 



The distance from A or B is found by subtracting the elevation of 

 either A or B from this quotient, and dividing the result by the rate 

 of grade. Or the summits may be located mechanically by specially 

 prepared scales. Prepare two scales divided to correspond to the rate 

 of grade; that is, if the rate of grade be 1 foot per 100 feet, then one 

 division of the scale should equal 100 feet on the map scale. These 

 divisions may be subdivided into tenths. One scale should read from 

 right to left, and one from left to right. 



To use the scales, place them on the map so that their figures 

 correspond with the corner elevations; then, as the scales read in op- 



Curb Le^el 



_ -* 0oTtom of Gutter -* 



Fig. 49. 



posite directions, there is of course some point at which the opposite 

 readings will be the same: this point is the location of the summits; 

 and the figures read off the scale its elevation. If the difference in 

 elevation of the corners is such as not to require an intermediate sum- 

 mit for drainage, it will be apparent as soon as the scales are placed 

 in position. 



When an accommodation summit is employed, it should be form- 

 ed by joining the two straight grade lines by a vertical curve, as 



