571 



METEOROLOGICAL CORRECTIONS. 



ONE of the prominent objects of a prolonged series of meteorological observations 

 is to determine the mean condition of the atmosphere, during a given interval of 

 time, such as a day, a month, or a year, as to its temperature, moisture, and baro- 

 metric pressure. In order to furnish the true means of these elements, free from the 

 periodic changes which depend upon the daily course of the sun and upon the seasons, 

 the observations ought to be made at equal intervals of time, and be so often repeated 

 as actually to represent the sum of the variations which took place during the stated 

 time. It is generally admitted that observations taken at every one of the twenty- 

 four hours of the day give means which do not sensibly differ from the means 

 which would be obtained from a still larger number of observations during the same 

 time ; so that means derived from hourly observations may be considered as the true 

 daily, monthly, and annual means of the year in which the observations were 

 taken. 



However, as the means of a given month, or year, wilt generally be found some- 

 what to differ from those of another year, at the same place, from causes which are 

 not of a periodic nature, it is obvious that the absolute means can only be derived 

 from the means of a series of years, in which the differences arising from these non- 

 periodic variations may be considered as sufficiently balancing each other. 



Hourly observations can be expected only from a very few stations, favored with 

 peculiar arrangements for the purpose. By far the larger number of observers must 

 necessarily confine themselves to three or four observations a day. The means, 

 therefore, deduced from such a set of observations, generally differ from the true 

 means which would be given by hourly observations, by a quantity which varies 

 with the hours selected for the observations. If that quantity, however, is known by 

 having been previously determined for every hour, or set of hours, by a long series 



F 7 



