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 repeated 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 extended 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 beforehand 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 number 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 observations.
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 difference 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 temperature 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
