Messrs. Editors:--In reply to "Spectrum," page 358, of last volume, I will be brief. In his third paragraph he claims that he has merely suggested that friction and percussion may often be one and the same thing; and immediately claims that in the case of the polished button rubbing a planed pine board, the force which overcomes and levels the undulations of the wood, is percussion, and that percussion is also the cause of the heat; the microscopic hills and hollows on the shining brass button skipping and jumping along the pine, produces little infinitesimal bumpings, and so pound out the heat. This little theory should be known to the homeopaths--they could illustrate infinitesimal quantities by it!

"Spectrum" treats my hammered horsenail illustration shabbily. After indirectly acknowledging that there is a point where hammering will no longer produce heat, he puts it on the grindstone, subjects it to friction, and when it burns his fingers, throws his hat in the air and shouts "Hurrah for percussion!" We agree perfectly, except that he calls hammering, condensation; calls friction, percussion; and drops friction from the mechanical dictionary altogether.

A railway car axle often heats and sets fire to the packing, when the journal is smooth as polished glass; but I never heard of those parts of the car which are constantly undergoing percussion, even getting uncomfortably warm. The natives of the South Pacific produce fire by rubbing pieces of dry wood together, but I never heard of their rapping sticks for the same purpose. I have seen a new, sharp knife made hot enough to raise a blister, whittling a clean dry stick of pine, and I would like to have "Spectrum" tell us, if in all the above cases percussion is the cause of the evolution of of heat, and what is friction doing in the mean time.

New Albany, Ind.

C.C.H.