2 INTRODUCTION. 



definite laws of the rain-fall, such as he has exhibited by curves for different sec- 

 tions of the country. The rain-fall, which apparently is one of the most irregular 

 of atmospheric phenomena, is nevertheless referable, from numerous observations 

 through long periods, to definite phases of occurrence, both in time and quantity, 

 and even the imperfections of the instruments, provided they give results in excess 

 as well as in deficiency, in a measure compensate each other. 



This memoir is one of a series embodying the results of all the labors of the 

 Smithsonian Institution in regard to the meteorology of the United States. These 

 will include not only all the observations which have been made under its own 

 direction, but also the discussion of all that have been made by other parties. The 

 whole series will embrace the tabulation and discussion of observations on the tem- 

 perature, atmospheric pressure, direction and force of the wind, moisture of the 

 air, and miscellaneous phenomena. The discussion of the temperature and the 

 winds, which has been in progress for several years, is now well advanced, and will 

 be published in part during the year 1872. 



Since the completion of this memoir an effort has been made to increase the 

 number of observations, and for this purpose a simple form of rain-gauge has been 

 adopted, and of this about four hundred have been distributed to different parts 

 of the country. 



The publication of this memoir will, it is hoped, tend still further to awaken 

 attention to this subject, and to secure a still larger number of voluntary observers, 

 which will enhance the value of the records, since this depends both on the num- 

 ber and on the duration of the series. 



JOSEPH HENRY, 



Secretary S. I. 

 Washington, D. C, 1811. 



