240 TENTH ANNUAL REPORT OF 



If an observer desires to examine the daily oscillations of the barom- 

 eter he will also observe at 10 a. m. and 4 p. m., which give the 

 dail}'' maximum and minimum. It will be well to note also, at the 

 same time, the state of the hygrometer. 



If he desires to complete the data upon the diurnal course of the 

 temperature, he will add observations of the thermometer at 10 a. m., 

 and 6 p. m. In all cases it is desirable that, if an observer has leis- 

 ure to increase the number of the hours of observations, he should 

 fix them at equal intervals between the principal hours indicated 

 above. 



Besides these observations at regular hours, additional observations 

 ought to be made during remarkable storms, as has been remarked 

 above. 



It is very important that the observations should be made at the 

 exact hour, fixed by a well regulated watch. All tLc instruments 

 should be read rapidly, so that the observations may be as simulta- 

 neous as possible. 



The order in which they are to be observed will be as follows : 



A few minutes before the hour, observe the thermometer before 

 opening the window ; then wet the psychrometer. While it is taking 

 the temperature of evaporation, note the height of the barometer, ob- 

 serve the -^ind, the course of the clouds, their quantity, the aspect of 

 the sky, &c. ; then read the temperature of the psychrometer. 



The observations must be recorded for each instrument at the mo- 

 ment when they are made, without trusting anything to the memory. 

 A strict rule should be laid down for one's self, to note exactly the 

 indications of the instruments, without subjecting them mentally to 

 any corrections or any reductions ; these should not be applied until 

 all the elements are at hand. 



If the observer has been unavoidably hindered from making the 

 observations at the exact hour, he will note in the column of hours 

 the number of minutes of the delay. If he is obliged to procure a 

 substitute, he must choose one accustomed to this kind of observation ; 

 but before entering his records, he will carefully examine them. To 

 distinguish the observations made by his substitute, he will enter 

 them in red ink. 



As it is of the greatest importance that the series of observations 

 should not be interrupted, and that there should be no omissions, 

 each observ'cr will do well to instruct beforehand one or more substi- 

 tutes, who may be able upon occasion to take his place. If, in spite 

 of these precautions, the observation has necessarily been omitted, its 

 place will be left blank in the journal. In this case the observer must 

 never fill up these blanks with calculations, according to his judg- 

 ment ; he should consider the conscientious observance of this rule 

 indispensable to truth and good faith. He should remember, besides, 

 that if he acts differently, he not only lessens the value of these re- 

 sults, but brings into doubt and disfavor the fidelity of his other ob- 

 servations, and takes from them what constitutes their greatest value 

 for science — confidence. 



