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1888 - 89 .] Dr R. W. Felkin on Tropical Diseases. 
should he come within reach of an epidemic. New arrivals are 
most exposed to this disease ; should they escape it at first they are 
the less liable to suffer from it, the longer they reside in one place ; 
but if they travel about, this acclimatisation appears to he lost. 
Whatever may he the real cause of yellow fever, its origin seems 
to he connected with heat, for although it occurs in sporadic and 
epidemic forms at all seasons in the tropical part of the yellow fever 
zone, the disease is greatest, and takes an epidemic spread at the 
hottest period of the year, a temperature of at least 70° F. being 
required for its production. Frost puts an end to an epidemic at 
once, and storms, heavy rains, or cold weather check its progress. 
Although heavy rainfall will stay an epidemic, yet moisture in the 
atmosphere would seem to be necessary for the production of the 
poison, for in dry years or during seasons of long-continued drought 
the number of yellow fever cases are always remarkably few. 
Winds influence yellow fever by their effect upon the atmosphere, 
but it does not appear that they are capable of conveying the 
poison, at least to any distance. On looking at the chart, it will 
at once be seen that yellow fever is most prevalent on the sea coast 
and along the courses of the great rivers ; it always spreads from 
centres of dense population, and the greatest number of cases 
occur in the dirtiest and most overcrowded parts of large towns. 
Altitude, certainly, has an important influence on the spread of 
yellow fever, and it is only in very severe epidemics that it leaves 
the plains. The protection which altitude confers against the 
disease is almost certainly due to the lower temperature of elevated 
spots, because where yellow fever has made its appearance in 
highly situated regions, it has always been in localities noted for 
the exceptional heat of the days. 
A certain saturation of the atmosphere is an essential condition 
for an epidemic of yellow fever. It is probable that it does not 
occur until a high dew-point, the minimum being upwards of 
74, exists, and it is certain that epidemics cease before the 
dew-point descends to 58. The geological characters of the 
soil have apparently nothing to do with the production of yellow 
fever, and all those conditions of soil which we found to be necessary 
for the production of the malarial poison, exert no influence in pro- 
ducing yellow fever. Electricity has a curious influence upon 
