CONSIDERATIONS ON THE OBJECT AND METHODS. 3 



values to the coordinates, we can examine the effect of letting time vary; or (2) 

 giving a constant value to time, we can examine the effect of letting coordinates 

 vary. These two different ways lead to two essentially different branches of 

 meteorological and of hydrographic science. 



89. Climatological Method. First let us give constant values to the coordi- 

 nates, and examine the effect of letting time vary. We can imagine the investi- 

 gation performed in the following way : Self-recording instruments are set up at a 

 number of fixed points (stations) in atmosphere or hydrosphere. The different 

 records of the meteorological or hydrographic elements then show directly the effect 

 of letting time vary, while the coordinates have the constant values defining a 

 certain station. 



When we examine the records we find great irregular changes, the explanation of 

 which can not be found by a direct examination of the curves; but conspicuous 

 signs of regular changes are also discovered. Forming averages in different ways, 

 the irregular phenomena will more or less disappear. The regular ones will then, 

 for the most part, present a periodical character, having the periods of the solar 

 day, of the solar year, of the sunspots, and perhaps of still other cosmic phenomena. 

 Besides the decidedly periodic phenomena, slow secular changes may also be dis- 

 covered. 



The different kinds of averages thus formed of the meteorological or hydro- 

 graphic elements may be called the climatological elements for atmosphere or 

 hydrosphere. Inasmuch as time enters into the definition of these elements, it is the 

 local time of each station, not universal simultaneous time. The elements found at 

 the different stations may be compared to each other. This leads to the drawing 

 of climatological maps, showing the average influence of geographical data, just 

 as the single curves showed that of astronomical events; but no way leads to the 

 investigation of the nature or the causes of what we called irregular phenomena. 

 These were eliminated, and to investigate them we must follow another way. 



90. Dynamic Method. In order to examine the other method, we can start 

 with the records obtained from the same set of self-recording instruments, but 

 shall make a modified use of them. Giving time a certain constant value, we read off 

 from all records the values of meteorological or hydrographic elements at this epoch, 

 and draw continuous synoptical representations of the field of each element. 

 Having thus got a complete picture of the state of the atmosphere or the hydro- 

 sphere at this epoch, we give time a new constant value, read off the new values of 

 the elements, and produce new synoptical representations of the fields, which give 

 a complete picture of the state of atmosphere or hydrosphere at this second epoch, 

 and so on. 



A series of such pictures being produced, the next step will be to make them 

 the subject of a comparative investigation. This comparative investigation of the 

 successive states must lead to the solution of the ultimate problem of meteoro- 

 logical or hydrographic science, viz, that of discovering the laws according to which 

 an atmospheric or hydrospheric state develops out of the preceding one. 



