476 Proceedings of the Royal Physical Society. 
Yellow Fever. 
Yellow fever is a pestilential contagious disorder of a 
continuous and special type, depending for its origin and 
spread on a temperature not lower than 70° F. As a general 
rule it occurs but once in a lifetime. Its spread is favoured 
by the gathering together of persons born in a cold climate. 
LEtiology.—Yellow fever is entirely distinct from malaria. 
Its production requires a temperature of from 68° to 70° F. 
When once originated, however, an epidemic may spread at 
a lower temperature, but it dies out if the temperature falls 
to freezing point. The influence of moisture in the air 
constitutes a second factor in the production of yellow fever. 
Abundant continuous rain does not infrequently bring an 
epidemic to an end, probably by modifying the temperature, 
but a certain saturation of the atmosphere is an essential 
condition for the production of the disease—probably 74 per 
cent. of moisture. Epidemics cease when the amount of 
moisture is as low as 58 per cent. The only influence which 
wind has on yellow fever is by its modifying the temperature. 
The disease rarely leaves the sea coasts and the shores of 
large rivers; it arises in the filthy quarters of towns, in the 
centres of poverty where the people are densely crowded. 
The geological characters of the soil have apparently no 
connection with the production of the disease. It is most 
interesting to notice the influence which circumstances of 
race, nationality, and acclimatisation exert upon the disease. 
Where it is endemic or epidemic, newly-arrived strangers, or 
such persons as have not yet become fully acclimatised, are 
the persons who suffer most. This is well seen if a large 
body of troops or a shipload of emigrants arrive at any place 
where the disease already exists, though it may be very 
mildly. An epidemic at once springs up, and the new 
arrivals are the first persons attacked. The degree to which 
this proclivity of strangers exists will depend to a great 
extent on their nationality, that is, on the mean annual 
temperature of their native country. The liability to attack, 
as well as the mortality amongst the newcomers, bears a 
close relation to the distance from the equator of their place 
