178 WRITINGS OF JOSEPH HENRY. [1855- 



tions. We may take, for example, the mean of all the tem- 

 peratures of mid-day for the month or the year, or of any other 

 hour of the twenty-four, and from this obtain the mean tem- 

 perature of all hours of the day and night. Finally, instead 

 of limiting our observations to a single year we may extend 

 them to a series of years, in order to determine more ac- 

 curately the mean temperature of a given place, all acci- 

 dental variations of particular years and seasons being reason- 

 ably supposed to balance each other. It is by this admira- 

 ble invention of extended averages that order and regularity 

 are deduced from phenomena which appear to be under the 

 influence of no fixed laws, and that we are enabled to arrive 

 at permanent and constant quantities, by eliminating those 

 which are irregular and variable. 



A series of observations continued during the day and 

 night through a number of years, or even a single year, in- 

 volves an amount of labor which few men of science can 

 afford to bestow upon meteorology; and few have the indus- 

 try and perseverance necessary to so prolonged and tedious 

 an effort. This task however has been performed under the 

 direction of several persons in this country, namely, Prof. 

 Dewey, in Massachusetts; Capt. Mordecai, at the United 

 States Arsenal, near Philadelphia; Prof. Bache, atGirard Col- 

 lege, Philadelphia; Prof. Snell,at Amherst; and Col. Lefroy 

 of Toronto ; not to mention the names of a large number of 

 persons who have executed the same work in Europe. 

 Could it be repeated in a number of different places in this 

 countr}^ the results would be of essential importance in 

 correcting the ordinary observations made at fixed hours of 

 the day. 



To illustrate these observations and the uses to which they 

 may be applied, we shall select a series made since 1816, at 

 the Observatory of Paris, by M. Bouvard, at six different 

 epochs of the day, namely, from nine o'clock till mid-day, 

 and from three to nine in the evening, the other hours be- 

 ing given by interpolation : 



