IMPORTANCE OF VARIABILITY 95 



The table indicates that on the whole, variability of temperature 

 IS a decided stimulus. 



Follow the figures through from January to December. From 

 January to the last of May there is an almost steady rise from 

 2.3 per cent to 6.0 per cent. Periods of good health evidently 

 follow times when the variability is above the normal for the 

 season, while the worst health follows times when the variability 

 is less than the normal. This tendency increases until the end 

 of May. That is, it is at a maximum when the temperature is 

 close to the optimum. Then during June and July the value of 

 changes in the weather declines. This is natural, for although 

 there is much less variation from day to day in hot weather than 

 in cold, the hottest days are included in the periods when the 

 weather is subject to the greatest changes. Nevertheless, even 

 in hot weather people's health is better when there is much change 

 from day to day than when there is little. Of course in the New 

 York climate there are never any long periods with practically 

 no changes of temperature, so we cannot tell how much greater 

 the value of variability would appear if we could compare people 

 who live where there are no daily changes with those who live 

 where there are many. 



Passing on to August we find that as soon as the hottest 

 weather is past, the value of daily changes increases to the maxi- 

 mum for the whole year. That is because the variable periods 

 are the ones in which cool waves begin to be prominent. Then 

 during September the value of variability appears to decline, and 

 from the middle of October to the middle of December the most 

 variable weather is not quite so favorable as that which is more 

 nearly uniform. The reason is not apparent. It should be noted, 

 however, that this is the time when people's health is at its best, 

 as is evident from the New York curve in Figure 7. Perhaps 

 when other causes combine to produce good health, variability 

 ceases for a while to be of so much importance. The figures with 

 the minus sign in the table are so small that they might almost 



