558 PRACTICE OF PHYSIC. 



" I proceed, accordingly, to point out the effects of the sensi- 

 ble qualities of the air, viz. heat and cold, dryness and mois- 

 ture, with respect to epidemic diseases. I refer diseases chiefly 

 to the first two, viz. the powers of heat and cold ; for, though 

 dryness and moisture seem to have a considerable share, I think 

 they will be found to be concerned only hi as far as they increase 

 or diminish the other two. Thus it is generally observed, that in 

 Britain the moist summers, being the coolest, are most free from 

 disease ; and Wintringham and Huxham observe, that moist 

 winters, being the warmest, are also the most healthy, providing 

 they be free from contagion, which the moisture seems to propa- 

 gate and to continue. This then is the general view, but there 

 are some seeming exceptions to it ; for though moisture certainly 

 always moderates the intensity or the degree of heat, if it does 

 not moderate it so as take it down below the putrefactive degree, 

 it does mischief. Thus, if the ordinary temperature of a climate 

 is at 60, miasmata may be produced in autumn ; but if the 

 season is moist, and the heat brought down to 50, which is 

 hardly compatible with any degree of putrefaction, such seasons 

 will escape epidemic diseases. But if the heat of the climate 

 is at 80, and the moisture takes down the heat below 70, this 

 is still sufficient to bring on putrefaction, and more miasmata 

 will be produced in consequence of the moisture. Heat is the 

 chief cause of the production of miasmata, but a degree of 

 moisture is necessary for then- accumulation, and the moisture 

 also contributes in applying them to the bodies of men. And 

 though in winter moist seasons be warmer than dry, they may, 

 by favouring evaporation, increase the force of miasmata. So 

 much for the effect of moisture in modifying the operation of heat ; 

 and although dryness increases the degree of heat, and increases 

 some of its effects upon the bodies of men, yet, by binding up of 

 the surface of the earth, by diminishing the exhalations, and by 

 diffusing them through the atmosphere, it prevents the genera- 

 tion and application of the noxious effluvia, and so may give the 

 most healthy season. The particular effect of the dry north- 

 east wind, on the contrary, upon men in most of the countries of 

 Europe, in exciting intermittent fevers, is not owing to any par- 

 ticular matter that it brings along with it, but to its being ex- 

 tremely dry, and, therefore, by a more sudden evaporation from 



