5 o6 POPULAR SCIENCE MONTHLY. 



barometric pressure, rainfall, etc., are made and recorded. We may 

 use one of two methods to examine them. The first is to group the 

 observations in various ways until we discover some group or groups 

 into which the observations appear to fall. This method, however, 

 entails such an enormous amount of labor that it has rarely been 

 adopted. It is only successful in the few cases where the general 

 nature of a group appears without much trouble. The second method 

 is to assume that the observations must run in series which are re- 

 peated after definite intervals of time. These intervals of time are in 

 reality obtained from the general physical principle that if any force 

 is periodic, that is, if its fluctuations are repeated after a definite period 

 of time, then certain of its effects are also repeated after a definite 

 period of time, which is the same as that of the force. The first method 

 is an attempt to construct the bridge by starting from one bank and 

 building piers or supports on which the structure may be gradually ex- 

 tended across the chasm; the second method resembles a series of 

 cables thrown across in the hope that they may be attached firmly 

 enough on each bank to bear the weight of the bridge. 



There is no difficulty whatever in attaching two of these cables to 

 both sides. Whatever other causes operate, there can be little doubt 

 that the sun must play some part in governing the climate of any 

 particular place. The day and the year are therefore marked out be- 

 forehand as periods into which observations of temperature can be 

 grouped, and these periods are fully confirmed. If the temperatures be 

 recorded every hour out of the twenty-four for a large number of days, 

 and the average temperature for each hour be formed by adding all 

 the observations for that hour and dividing by the number of them, we 

 obtain a series of twenty-four average temperatures. When the num- 

 ber of days on which observations are recorded is great enough, these 

 averages will show a regular change rising to a maximum, for most 

 places, in the early afternoon and descending to a minimum in the 

 early morning hours. A similar method followed with the average 

 temperature, say, for each day, will show a yearly period in the observa- 

 tions. 



We thus get a series of average temperatures for each hour of the 

 day and for each day of the year. But these are only averages and 

 they only present regularity when very large numbers of observations 

 have been used in forming them. At any particular time the differ- 

 ence of the actual temperature from the average temperature for that 

 time may be as large or larger than the greatest difference of the 

 averages during the period. It is evident then that other cycles must 

 be sought which, by their combined effect, will give the actual tempera- 

 ture at any particular time. And what has been said about temperature 

 can be said to a greater or less degree about the other phenomena, such 



