111=31,35; 114=31,52; 116=32,45; 119=33,9 u.a. von: Iowa State College: Official publication
190=19; 191=20; 192=21; 193=22; 194=23; 195=25; 196=26 von: Iowa. Highway Research Board: Bulletin
139=36,42; 140=36,48; 141=37,51; 142=37,52 u.a. von: Iowa State College: Iowa State College bulletin
186=58,29; 187=58,39; 189=59,24; 190=59,26 u.a. von: Iowa State University: Iowa State University bulletin