214 PROCEEDINGS OF THE ACADEMY OF [18H7. 



abruptly to the southwest, when one or two hundred feet from the 

 heavy masonry of the canal locks. 



The exposed portions consist of heavy timbers and planking ; the 

 top and a lower section being horizontal or nearly so, connected by 

 an inclined slide or shoot. To increase the depth of the water above 

 the dam, for the benefit of the Philadel})hia Water Works, a series 

 of planks have been hinged to the up])er, horizontal portion, and 

 are retained in a perpendicular position, by inclined stays or props. 

 When the river is full, the water stands against these and overflows 

 them, nearly two feet above the normal level of the dam. Durbig 

 the summer season, however, the diminished supj)ly is so drawn 

 upon by the operation of the canal locks, the water works and ordi- 

 nary leakage, that there is rarely any overflow for several months 

 together. At such times it has been my custom frequently to walk 

 fr()m one side of the river to the other upon the horizontal timbers, 

 examining at leisure the inner (uj^per) side of this planking ; and 

 my collections have principally been made from their smooth 

 surfaces, when, illuminated by the afternoon sun, every adherent 

 growth was easily seen through the unruffled surface of the water. 

 Not that the stream is perennially unruffled by any means, for at 

 its best the wave from a jjassing steamboat is likely to flood the un- 

 wary scientist at an unexpected moment. I only allude to this 

 quiet hour, with the declining sun nearly in front of the ex[)h)rer, 

 as presenting the most favorable conditions for collecting. Upon a 

 single visit, I have gathered from these timbers, specimens of»S'. 

 lacustris, S. fragilis, M. leidyl, M. JiuviaUlis and 3f. crater if ormis, 

 with minute portions of H. argyro.^perma. The first three may be 

 considered permanent inhabitants of the locality, the others as occa- 

 sional visitants. 



In the forebay of the water works and in the reservoirs above, 

 M. leidyi is very abundant, covering all standing timbers, horizon- 

 tal pipes and frame work, as well as the dressed stone facings of the 

 piers etc. In tide water in both the Schuylkill and Delaware Rivers, 

 it is found at the greatest depth examined, say eight feet 

 below low water, and probably grows much deeper ; while most of 

 our other sponges seem to prefer the neighborhood of the surface. 

 In the subsiding reservoirs upon FairmountHill and in other places 

 about Philadelphia, it especially aflects the cribs and screens at the 

 head of the discharge pipes. When the water has been drawn oft 

 for cleaning I have found it covering yards of su-rface at such places, 



