igoy ] Proceedings N. C. Academy oe Science. 47 
wall, shown to be too high and therefore on theside of safety. 
But since the pressure near a face, acts necessarily parallel 
to that face, the vertical unit pressure just computed, is not 
the whole pressure. 
The difficulty of computing exactly this whole pressure is 
next entered into and an upper limit found by an approximate 
method which again gives an excess pressure. 
Rankine’s suggestion, to use the ordinary formula for ver- 
tical unit pressure, but specify higher limiting unit pressures 
for the up-stream face than for the down stream face is adopted 
provisionally. 
The claim is made that, in addition to the three universally 
imposed conditions, no tension, safe unit pressures and no 
possible sliding at any horizontal joint — a fourth condition 
must be imposed, viz., that the factors of safety against 
overturning and sliding shall increase gradually from the 
base upwards to allow for the proportionately greater influ- 
ence, on the upper joints of wind and wave action, floating 
ice or other bodies, and especially of the great forces caused 
by the expansion of thick ice under an increase of tempera- 
ture and by earthquakes. 
It was found that this could easily be done by taking the 
well-known theoretical triangular type of cross-section of 
dam and making some additions at the top sufficient for a 
roadway. 
A preliminary design is given for a dam 258 feet high, 
with factors of safety and unit pressures marked on the draw- 
ing, satisfying all four conditions. The area of cross-sec- 
tion and height being the same as for the celebrated Quaker 
bridge design, a comparison was instituted, unfavorable to 
the latter, in that its factors of safety are too small, par- 
ticularly in the upper portions, where by the proposed fourth 
condition they should be largest. 
This criticism owes its significance to the fact that the new 
Croton Dam of New York, 224 feet high to water surface and 
finished February 1st, 1906, at a cost of over $7,500,000, has 
