BY E. M. JOHNSTON, F.L.S. 25 



the total death-rate for all ages witliout any disturbance of 

 matters affecting the local health and sanitary conditions. 



The foregoing influences may be described as the principal 

 disturbers of the death-rate index, although in no way 

 affecting the conditions connected with health and sanitation. 



The influences commented upon hereafter (viz., season, 

 climate, cosmical or epidemic visitations, war, famine and 

 violence, density of population, etc.,) form a group which 

 greatly affects both health and death rates for all ages, even 

 though the state of sanitation be good, or at any rate 

 normal, and therefore comparatively neutral as regards 

 death-rate variations. 



The Inflxjektce of Climate. 



Independently of all other causes enumerated, the death- 

 rates of different countries are seriously affected by special 

 diseases, which are more or less restricted to given geo- 

 graphical limits. Yellow Fever may be taken as an example 

 of diseases of this kind, being mainly restricted to certain 

 tropical latitudes, and there, again, mostly confined to low- 

 lying plains adjacent to -the sea.* 



The total death-rate for such places certainly may give a 

 fair index of its health from year to year ; but its presence 

 or absence in different countries is a condition of latitude 

 rather than local hygiene. 



* Hirsch states that in the Western Hemisphere its range extends 

 between 32°"46 N., and 22°*54 S., and in the Eastern Hemisphere between 

 14°-53 K, and 5°-7 N. 



The Influence op Season. 



The seasonal influence has an important bearing upon 

 health, although its influence is similar in different years. 

 Its effect, therefore, is seen in the varying monthly or 

 quarterly death-rates rather than in the total yearly death- 

 rate. 



If we take our own local experience of the influence of 

 each month, and reduce it to a diagraphic form, we 

 readily perceive that there is an intimate relation between 

 the temperature and the death-rate. 



These temperature extremes are in January and July, the 

 first showing the maximum, the second the minimum. ISTow, 

 extremes of heat and cold are both injurious to health, 

 although, upon the whole, the former is more fatal. The 

 death-rate curve for the year sensitively follows the double 

 curves of temperature, although the second curve occurring 

 between April and November appears in inverse order. Thus 

 the double death-rate curves and double temperature curves 

 run together (between November and May, and May and 

 November), the maxima of the two death-rate curves closely 



