STADIA MEASUREMENTS. 93 



tain theodolites and telescopic alidades are equipped with stadia wires. 

 These wires are three in number, the intervals between them being equal. 

 The rods are 14 feet in length and hinged so as to close to 7 feet. The 

 intervals upon the rods are of one foot each. The wires in the telescope are 

 so spaced that when the rod is at a distance of 100 feet, the space between 

 the two extreme wires will subtend one foot on the rod. At a distance of 

 1,400 feet, therefore, this space will subtend the entire length of the rod, 

 while at a distance of 2,800 feet two adjacent wires in the telescope will 

 subtend the entire length of the rod. Distances less than 100 feet are esti- 

 mated by means of the fractional part of a foot upon the rod, which is 

 included between the wires. The distances are read off upon the rod by 

 the surveyor at the instrument. 



In measuring distance upon slopes, correction must be made to reduce 

 the inclination measured to horizontal distance. Tables for this reduction 

 are to be found in Bulletin. Where the slope is slight it is not regarded as 

 necessary to make_ this reduction, especially where there are frequent points 

 for checking and correcting the line. 



The rod may be used also for measurement of the profile of a line. 

 For this purpose, a point should be marked upon it at the same height as 

 the telescope of the instrument and vertical angles taken to this point. 



The work which has been carried on in southern Louisiana is peculiar 

 in the fact that the slopes are extremely gentle, requiring, in order to show 

 the relief at all, a contour interval not g-reater than 5 feet. For the location 

 of contours of so small an interval, even vertical angles are not sufficiently 

 accurate, and the work of measurement is effected by spirit level. The 

 instrument used is a theodolite of compact and simple form, to which the 

 name of gradienter has been applied, which is equipped with stadia wires. 

 The low ridges which accompany the streams of this region and which form 

 all the relief are located by means of lines run approximately at right angles 

 to the streams from their banks down to the swamps on either side. Dis- 

 tances are obtained by stadia and differences of elevation by using the 

 gradienter as a wye level, and the stadia rod as a level-rod. 



