VI PREFACE. 



covered. On tlli^< account the tal)les have been so constructed as to admit of con- 

 tinued additions and corrections. 



Tlic degree of 2)recision of such observations, made, as they have been, by so 

 manv jiersons of different kiiowlcdnc and cxpericMice, must, of course, vary within a 

 consi(k'rably wide range; still it would appear from the results of the discussions of 

 Mr. Schott that they are suificiently exact to attord satisfactory evidence of definite 

 laws of the rain-fall, such as lie has exhibited by curves for different sections of the 

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

 plienomena, is nevertheless referable, fi"om 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 iu 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 pai'ties. 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 Avill be pub- 

 lished 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 number 

 and on the duration of the series. 



JOSEPH HENRY, 



Secretary S. I. 



Washington, D. C, 1871. 



