IN RAIN AND SNOW IN THE UNITED STATES. xv 



• measuring-rod divided into inches and tenths. Instruments of the kind last men- 

 tioned are now being distributed among observers. 



To measure thousandths of inches is considered useless labor, since no two 

 gauges can be made to give results agreeing in the hundredths of inches, owing 

 to the irregularity in the rain-fall itself. 



EXPLANATORY REMARKS ON RECORDS. 



The value or precision of the observations collected from such various sources 

 must necessarily have a wide range, and while the greater portion is supposed to 

 come up to a standard precision, a smaller portion partakes more or less of un- 

 certainty, either from want of reliable instruments or from defective observing. 

 In a phenomenon so very irregular as that of the rain-fall, it is difficult to separate 

 defective observations from correct ones ; rejections of observations have, therefore, 

 been resorted to very sparingly, and only in such few instances when other records 

 have most undoubtedly proved their unreliability. It may be well briefly to enume- 

 rate the principal sources of error to which observations of the rain-fall were found to 

 be liable : A defective construction of the gauge ; error in the graduation or scale ; 

 improper location of gauge in reference to surrounding objects ; and allowance, by 

 rule, for water fallen as snow, instead of measuring the melted snow (or hail). Rain- 

 gauges as commonly used, may be supposed liable to no greater error than about two 

 per cent. ; this limit includes error introduced by a defect in the graduation or in the 

 manner of applying a measuring-rod. The most frequent cause of difference in the 

 results by two observers at adjacent stations will generally be found in the manner of 

 exposure of their gauges ; the proper location with reference to level ground and sur- 

 rounding objects, as trees, houses, or hills, being left to the judgment of observers, the 

 rule has not always been observed to place the gauge horizontally at a distance from 

 any such object of at least twice the amount of its elevation above the location of the 

 gauge. The eifect on the wind of elevated objects is to produce eddies, often felt at 

 great distances, as may frequently be seen by the whirling motion of dust. The rule 

 generally adopted of allowing one-tenth of the depth of snow as its equivalent amount 

 in water, is, of course, a very rough one, as the allowance must depend upon the 

 temperature of the snow, the form and size of the snowflakes, and the depth fallen, 

 since the snow will be compressed by its own weight. For precise nieasux'es, the 

 water resulting from the melted snow should always be recorded. The loss by evapo- 

 ration of the water collected in the gauge, or of the snow when melting, can easily be 

 guarded against. Respecting the elevation of the gauges above the general level of 

 the place, so very little information could be obtained that no special mention of it is 

 made in the tables ; they were generally on the ground, or at most, but a foot above. 

 There can hardly be too many observers even within a limited area, for instance. 



