Of the imagination, as a cause and as a cure of disorders of the body; exemplified by fictitious tractors, and epidemical convulsions. Read to the Literary and Philosophical Society of Bath.

The first clinical demonstration of the placebo effect, specifically in the context of Perkins' metallic tractors. Haybarth  demonstrated the placebo effect caused by the tractors by obtaining the same results with wooden ones. Digital facsimile from the Internet Archive at this link.