of the plate 35 39 to engage and disengage a ferromagnetic block 37 39 carried on top of the subjacent scraper blade 26 39 the operating bar 40 slides vertically in the channel defined by the depending portions 27b 39 and 34b 39 the top of the bar 40 39 is beveled as before for engagement with the lowering rampe 39 39 in the present modification the lower end of the riser 40 39 is coplanar with the hard rubber insert 26a 39 of the scraper blade 26 39 and the risers 40 39 are so located as to register with the lifting ramps 38 39 at the right hand end 22 39 of the basin thus the bottom ends of the risers 40 39 engage the lifting ramps 38 39 near the end of the sediment collection and removal pass and serve to elevate the scraper blade by making the risers of low friction material such as a wear resistant 34 nylon 34 both the lowering of the blade at the end of the return pass and the lifting of the blade at the conclusion of the collecting and removal pass is smooth a second modification is illustrated in fig1 in which double primes are used to designate elements previously disclosed in the second modification the magnet and ferromagnetic block combination is replaced by a spring clip 81 and pin 82 provided with a head 83 capable of prying apart the spring clip arms 84 when moved into engagement therewith and of.