120 TABLES AND RESULTS OF THE PRECIPITATION, 



GENERALIZATION OF TABULAR RESULTS AND CONSTRUCTION OF RAIN- 

 CHARTS OF THE UNITED STATES. 1 



The mutual relations of the preceding tabular results can best be shown by a 

 graphical representation, and I therefore proceed at once to the explanation of the 

 .three rain-charts accompanying this paper, which exhibit the most obvious results 

 of the co-ordination. 



Construction and Analysis of the Bain-Charts of the United States. — From the 

 material collected in the general table of results, the geographical distribution of 

 rain over the area of the United States is shown on the accompanying three charts, 

 for the average amount fallen during the year as well as for the summer and winter 

 seasons. For other parts of the Western Continent the observations are too scanty 

 even to attempt a graphical representation. 



To explain the construction of these charts it suffices to show it for the one 

 exhibiting the annual amount. All stations at which the observations extended 

 over four years and more, were plotted by their co-ordinate latitude and longitude, 

 and against the dot was written the nearest full inch of the amount of rain (and 

 melted snow) ; curves were then drawn with a free hand among these dots, for a 

 certain amount of rain, uniting all places at which this amount is recorded. These 

 curves, designated as isohyetal lines, and constructed in the manner of contour lines 

 generally, are given for equal intervals of four inches, an amount which resulted 

 from a consideration of the probable error of the results. If drawn too close, the 

 curves exhibit temporary or accidental inflections which only tend to confuse, 

 and if drawn too far apart there is danger of losing part of the permanent features 

 of these rain-curves. Next, all stations, having between one and four years of record, 

 were plotted and marked with a cross, and the amount noted as before ; these latter 

 numbers were only used to complete, modify, and generally improve the curves 

 previously constructed, and have of course less weight than the numbers first plotted. 

 The increasing amount of rain, as shown by the increasing figures written against 

 each curve, is also indicated by somewhat heavier lines. This chart exhibits the 

 results from nearly 790 stations. Those for the extreme seasons depend nearly upon 

 the same number. 



General Character of the Distribution of Rain on the Average during the Year. — 

 The most striking features of the phenomenon of rain, as delineated on the chart, 

 are the apparent precision and continuity in the law of its distribution, and the great 

 variation or range in its amount. Thus the curve, passing over places where the 

 annual fall amounts to 40 inches, can be followed from New Brunswick, on the Bay 

 of Fundy, to Texas; the isohyetal line of 36 inches, similarly, runs without inter- 

 ruption from the St. Lawrence River to the mouth of the Rio Grande. The regu 

 larity of the curves is sufficiently distinct to mark out everywhere the progression 

 in the deposition of the aqueous vapor. The annual amount varies from four inches 

 in the Yuma and Gila Deserts, at the head of the Gulf of California, to 80 inches 



1 For the original skeleton maps the Institution is indebted to the U S. Coast Survey. The 

 projection used is that known as the "polyconic," explained in the Coast Survey Report for 1859, 

 Appendix No. 33, or Report for 1853, Appendix No. 39. 



