TBI ANGULATION AND COMPUTING. 67 
graphic survey, and this would have cost immense sums and occupied 
several years, during which the topographic surveys would have con- 
tinued without being accompanied by spirit leveling and the establish- 
ment of the bench marks required. The act was therefore so framed 
as to permit of the acceptance of some point within each area under 
topographic survey as a central datum point for that area, and the ele- 
vation of the initial bench mark established there was determined as 
nearly as practicable from existing elevations adjusted through by 
railway levels or other levels brought from the sea. In consequence, 
though all the elevations connected with the same central datum point 
agree one with the other, they may not be reduced to mean sea 
level because of the differences between the primary elevations upon 
which leveling in the various localities is based. However, prior to 
and since 1896 precise-level lines have been extended by this Survey 
and other organizations to more accurately determine inland elevations 
above sea level, and the elevations original^ determined have been 
corrected from time to time in publications, so that at the present 
time nearly all the central points have been reduced to mean sea-level 
datum, carrying with them all the levels resting thereon. 
It was decided in 1896 that in addition to determining the elevations 
of these bench marks they should be instrumentally connected with 
the horizontal measurements taken in the course of topographic sur- 
veys, but only with such accuracy as would permit of their being prop- 
erly plotted on the resulting- maps, the location and elevation of these 
bench marks being published in two ways — first, Iry a symbol on the 
atlas sheets, the letters B. M., and the elevation to the nearest foot; 
second, by lists of bench marks in the annual reports or bulletins of 
the Survey, with a full description of each bench mark and its exact 
elevation above sea level to the thousandth of a foot, as adjusted and 
referred to the various central datum points, these lists to be cor- 
rected in publications from time to time as better connections are made 
with sea level. 
It was thought that these bench marks should be of the most 
attractive and substantial character consistent with reasonable cost, 
and after an examination of the various forms used by other Govern- 
ment and city surveys, two standard styles were selected. First, 
a circular bronze or aluminum tablet, 3£ inches in diameter and one- 
fourth inch thick, appropriately lettered, having a 3-inch stem to be 
cemented into a drill hole in the vertical walls of public buildings, 
bridge abutments, or other substantial masonry structures. The 
second form, to be employed where masonry or rock is not accessi- 
ble, consists of hollow wrought-iron posts, 4 feet in length and 3^- 
inches in outer diameter, split at the bottom and expanded to 12 
inches, so as to prevent both the easy subsidence of the post and its 
being maliciously pulled out of the ground. The iron is heavily 
