Engraving of the week

'Ballasting'  From 'Life on the Upper Thames' 1875.

Inexplicably H R Robinson the author has nothing to say about this engraving other than to state that it was an operation carried out to maintain a navigable channel .
Before the era of steam dredgers this was an obviously laborious way  of dredging and seems to be a similar but even more primitive technique than the old spoon dredgers used on the canals.