﻿38 THE AIR OF TOWNS. 



The numbers represent averages tlirougliout the year, but this 

 includes considerable variations, which occur at different seasons of 

 the year. 



The shaded portion in the diagram (fig. 39 ) represents the number of 

 bacteria, and the dotted line the temperature during the various months 

 of the years 1879-1S82. 



The number does not appear to vary proi^ortionately with change 

 of temperature : but if we compare the rainfall with the number of 

 microbes we see at once a rapid diminution. The rain evidently car- 

 ries them down to the earth. But they are far from being destroyed. 

 The moisture seems to assist reproduction, for we find a rapid increase 

 directly after rain. If drought is long continued the number falls off' 

 again. They die. Here, again (fig. JrO), the shaded portion represents 

 the number of bacteria, and the line the rainfall during the year 

 1879-80. 



The number of microbes in the streets of Paris is on the average 

 about :2L to 22 in the cubic foot, and this agrees with that found by 

 Professor Carnelley in the streets of Dundee, viz, 20 in the cubic foot. 

 Outside of Paris the number falls off' to 2 whereas, in dirty, one-roomed 

 houses Carnelley found 3A'30 and Miquel in a neglected hospital ward 

 3,170 in the cubic foot. The effect of population in increasing the 

 number of microbes may be represented by the following rough map of 

 Paris (fig. 11), in which the number of microbes in a cubic meter of 

 air observed at Montsouris is marked against the arrow denoting the 

 direction of the wind. From this it will be seen that the largest number 

 occurs when the wind blows across the town and the smallest number 

 when it comes direct from the country — that is. from the south. 



The number, 21 to 22, for the streets of Paris is a rough average. In 

 dry. dusty weather, following rain, the number may rise to 150. Directly 

 after wind and rain it may fall to an average of 6 per cubic foot. 



T\'e can not be suri3rised that the washings of the air by rain, accu- 

 mulating in the mud^ of thoroughfares, should be the gathering ground 

 for microbes. The mud of streets is more than this. It i^rovides 

 food for their growth. It is the great source of bacterial propagation. 

 TThen we open our windows to let in fresh air on a dry, windy day, we 

 are welcoming these small visitors. The number of microbes in a grain 

 of dust from the streets of Paris was found to be 81,210, nearly double 

 that contained in similar dust obtained on the outskirts of the town. 



Can we be astonished at finding domestic dust nearly as pregnant 

 with living matter as that from the street, which, according to Miquel, 

 is 61,000 in the grain? It might appear judicious to keep our windows 

 closed under such a siege, but a moment's reflection will, I think, solve 

 the difiiculty. We do not know to what degree these microbes are 

 mischievous. We do know to what extent Iresh air is necessary to 

 health. Let us admit air, but keep our dwellings, as far as possible, 

 free from dust. Microbes settle rapidly in still air, and we have only 



