THE SMITHSONIAN INSTITUTION. 239 



Bainboivs, simple or double. 



An extraordinary redness of the sky, either in the morning or eve- 

 ning ; the particular color of the sun and of the moon at their rising, 

 especially in fair days. 



Heat lightnings without thunder, and sometimes without clouds ; 

 indicate their direction and the aspect of the clouds in their neigh- 

 borhood. 



The Aurora Borealis, or northern light, for the observation of which 

 the special instructions published by the Smithsonian Institution must 

 be followed. 



Shooting-stars. — The observer must be particularly attentive to 

 their frequency, during the periods near the 10th and 11th of August^ 

 and the 10th and 15th November, in which it is supposed that they 

 are more numerous than at any other time. He will designate the 

 quarter of the heavens from which they seem to issue, and their di- 

 rection. 



Fireballs. — Describe their aspect, their size, their course in the 

 heavens, and note the exact hour of their appearance. 



All the other luminous phenomena, which present any extraordi- 

 nary appearance, should be noted down. 



These descriptions should be made in simple and well-defined terms. 

 The observer will take great care to enter scrupulously ivhdt he sees, 

 without drawing any conclusion, or attempting any explanation of 

 the phenomenon. He ought to reflect that, in order to make a good 

 observation, he must keep his mind in a state of perfect disinterested- 

 ness in respect of any preconceived theory, and to consider the phe- 

 nomenon before him as being one of the data for the foundation of 

 the science, and that the knowledge of the truth will depend upon 

 the fidelity of his observation. 



TIME OF OBSERVATIONS. 



The time of observations will be the mean time at each station. 

 The observations will be made three times daily, viz : 

 At 6 o'clock, a. m. 

 .2 " p. m. 

 10 '' p. m. 



The mean of these three hours will be very nearly the true mean, 

 as it would be obtained by observation made every hour of the day 

 and night. They are at intervals of eight hours from each other, and 

 are the least inconvenient possible for the daily occupations of life ; 

 they must be preferred to any other series of three equidistant hours. 



[For convenience of observation the hours which have been adopted 

 by the Institution are Y, 2, and 9.] 



The ombrometer will be observed only once a day, unless very abun- 

 dant rains should make a second measurement necessary. The best 

 time will be 2 o'clock p. m., the observation being made daily ; if 

 another hour is selected, it should, when once fixed, remain the same. 



The maxima and minima thermometers will be read once a day, 

 always at the same hour. The most suitable hour will be 10 o'clock 

 in the evening. 



