The "Observationes medicae" and the first "Epistola responsoria" contain evidence of a close study of the various fevers, fluxes and other acute maladies of London over a series of years, their differences from year to year and from season to season, together with references to the prevailing weather, the whole body of observations being used to illustrate the doctrine of the epidemic constitution of the year or season, which he considered to depend often upon inscrutable telluric causes.