Statistics, which furnish a method of inquiry wholly unknown in Harvey's time, supply us with evidence of the course of epidemics, of their successive appearance in each locality, and of the gradual increase and decline of the several visitations. These facts reveal an unerring course of geographical direction which is taken by disease; and it is already clear that the explanation of this observed course, and of the mode of propagation, is to be looked for in the diffusion and interchange of gases during the existence of epidemics. From whatever cause arising, we can at least speak of an unbalanced state of atmospheric electricity; whilst we must refer the effect to a combination of forces not as yet clearly made out. Our two great philosophers, Grove and Tyndall, are continually contributing fresh discoveries on this important topic, which promises such fertile results to aid in the pursuit of medical philosophy. We also owe a great deal to the labours of Graham; and it is not possible to glance even casually at this subject without acknowledging the debt which meteorological science
