At first sight nothing seems more frail than a herring or mackerel net, one of those slight pieces of mesh-work that, in a continuation of lengths perhaps half-a-mile long, is let down into the sea to float with the tide, ready for the shoals of fish that dart against it as it forms a filmy wall across their way.  The wonder always is that it does not break with even a few pounds of fish therein, but it rarely does, for co-operation is power, and it is in the multiplicity of crossing threads that the strength consists.