History of Animal Plagues. 309 



want of a sufficient quantity of wholesome food, repeated alternations 

 of heat and cold in the weather, moist air replete with putrid vapours, 

 a long continuance of easterly winds, or, what is more frequent, a com- 

 bination of two or more of these causes together. Thus we lind this 

 contagion invading every part of Europe after the year 171 1, when the 

 season had been so inclement the year before as t© destroy a great por- 

 tion of the sheep in England j and again in 1741, as mentioned above, 

 after a very intense frost, which lasted from December to April, and 

 by the rigour of cold and scarcity of fodder, had reduced the cattle in 

 general to a debilitated state, which was still aggravated by the almost 

 constant easterly winds that blew the summer and autumn following. 

 After the abatement of it, in more clement seasons, which followed 

 that period, we find the disease resuming its force again in various 

 places, since that time, by similar accidents. We see, also, that from 

 the same principles it has long maintained its ground in countries where 

 moist air and vapour of stagnated water disposes to agues and other dis 

 eases that come from the laxity of the fibres, which disposition is equally' 

 produced by these causes, in the brute and human species. While, on 

 the other hand, it has been extinguished in places where a drier and purer * 

 air gives robustness and vigour of constitution to the animals, it had 

 been observed, also, that the infection, when it has abated during the 

 summer or winter, has generally resumed fresh vigour in the spring 

 and autumn, when the alternations of heat and cold have disordered 

 and weakened the beasts. 



' The same principle, with respect to the cause of the prevalence of 

 the contagion, holds good as to the particular beasts which are seized 

 with the disease, or escape the contagion when exposed to it. For 

 we see, in all cases, it is the weaker which are attacked by it, 

 and the stronger which remain free from it. It is ascertained, by 

 a very sufficient basis of remark on facts, that the beasts of a black, 

 dark, or red colour, either wholly or in mixture, are less subject to the 

 infection than those which are white, or of a light mixture of colours j 

 and the concurrent observation of all times has established the belief 

 that this difference of colour is an indication of the greater or less degree 

 of natural strength of the beasts, as well in neat cattle as horses. It is 

 equally certain that bulls are less subject to the infection of this disease 

 than oxen, oxen than cows, cows not pregnant than those which are 5 

 and, among all these, such beasts as appear naturally of a feebler make, 

 or are debilitated by any accidental suffering. 



' It is manifest, too, from an e(|ual ground of observation, that the 

 same difference in strength and weakness which renders the cattle 

 more or less susceptible of the infection, makes ihem more or less liable 



re / 



