INTRODUCTION. 139 
gates and gate frames fitted to stone jambs and lintels. The frames are faced 
with brass for the gates to work against. The gates are operated by a wrought- 
iron screw rod, with a brass nut working in a cast-iron socket. The water falls 
from the gates into a well, and is carried off through a culvert. ‘The waste-weirs 
are protected by stone buildings with brick arch roofs. 
The dam in the Croton river, as first constructed, was provided with a waste- 
weir 90 feet wide, which, in the high flood of January, 1841, proved insufficient 
to pass the water, and a breach was made in the embankment about 200 feet 
long. This breach was then filled by a structure of hydraulic stone masonry, 
adopting 180 feet thereof as an additional waste-weir. The greatest height of 
the weir of the dam is 40 feet above the low water mark, and 55 feet above the 
bed of the river. The width of masonry at low water line of the river is 61 feet. 
The form on the lower face commences on a curve described by a radius of 55 
feet, and continues to within about 10 feet of the top, when a reversed curve, on a 
radius of 10 feet carries the face over and meets the back line of the wall. The 
back line is carried up vertically, with occasional offsets. ‘The main body of the 
work is laid up of rough stone; the curve face of large and closely cut stone 
with four heavy courses at the bottom dovetailed together; the jomts cut to the 
line of radius of curve. Above the masonry an embankment of masonry is filled 
in in width 275 feet on the bottom, with a slope of 1 to 5 on the up-stream face. 
The north end of the new weir is terminated by an abutment which rises 12 
feet above it. 
From the toe of the masonry an apron is extended 35 feet, composed of hewn 
timber, well secured, and filled for 16 feet from the stone work, with concrete 
masonry; and the remainder with loose stone, and the whole covered with a 
course of six inch white elm plank. A second apron is made, extending 30 feet 
further. At 300 feet below the main dam is a second dam nine feet high, which 
sets the water over the apron of the main dam, and thus forms a pool to check 
the water as it falls over the weir. About 120 feet of the foundation of the dam 
is of concrete masonry, laid down on a very firm hardpan, and the remainder 
