The time-differential must therefore be chosen so as to suit the element which has
the most rapid time-variation. But nothing hinders elements having slower time-
variations from being observed, only, for instance, at every second or every third of
the epochs of observation, which have thus been chosen.
93. Special Remarks on Meteorological Observations. — In passing to concrete
meteorological observations, we shall first make some remarks regarding the principle
of simultaneity.
The ideal is of course the use of self-recording instruments having sufficiently
large time-scale. But whichever instruments or methods of observation be used,
it will be neither possible nor required to realize simultaneity in the mathematical
sense of the word. Most meteorological elements will under ordinary circumstances
change very Uttle dming as small an interval of time as, for instance, half an hour.
Departiu-es of this magnitude from the precise epoch of observation will therefore
not usually produce errors of greater importance, though exceptions are not excluded,

The general slowness of the variations makes it possible to use averages
registered during suitable intervals of time instead of true instantaneous values.
For one element, wind, the use of averages, as we shall see, will be tmavoidable, and
it will have certain advantages also in connection with other elements, especially
inasmuch as time-integrations should be performed afterwards. But if averages
be used, they should be used at all the cooperating stations, and taken according to
the same rules at all. These conditions have been excellently futfillecl by hoiu-ly
averages which we have obtained from the U. S. Weather Btu-eau.
