112? Mr, Davenport on Rain-Guages. [Feb. 



tances from each other out of a cloud of given magnitude 

 extending horizontally over a certain area of land, whether those 

 rain-drops fall perpendicularly, or proceed with a motion derived 

 from a projectile force received in the upper regions, the hues 

 described m their fall will be parallel to each other, and the 

 space they fall on on the surface of the earth will be in either 

 case equals to the area of the cloud : in one case, immediately 

 under the cloud ; in the other, at some distance from it. In 

 either case, similar rain-guages would receive equal quantities 

 with each other. This cannot admit of serious dispute; but 

 when we consider that the oblique fall of rain-drops is given 

 them by the motion of the medium through which they pass, 

 and that when that medium (the wind) with its rain-drops 

 impinge against any opposing surface, the rain-drops are 

 retained by that surface, while the volume of wind proceeds dis- 

 charged of that rain, the case is altered ; for it becomes evident 

 that an eddy of reverberated wind actually contains a smaller 

 quantity of rain than is contained in an equal volume of uninter- 

 rupted wind. 



Suppose now a wall extending on a plain, and the rain in its 

 obhque fall beating against the face of this wall, and two rain- 

 guages, placed one on each side of the wall at the base, and a 

 third on the top. If the oblique direction of the rain were 

 owing to an original projecting force, one of those at the base 

 would receive no rain at all ; but the other, and the one at the 

 top, would receive equal quantities with each other (putting out 

 of question the trifling difference oi accumulation oi dixo^^ falHng 

 through the small perpendicular space between them) and those 

 quantities would be equal to those of similar guages placed on 

 the uninterrupted plain. But since the oblique direction is 

 owing to the horizontal motion of the stratum of air through 

 which the rain passes, we ought to expect the upper guage to 

 receive less ; for the volume of air that beats against the wall is 

 not annihilated, but must rise ; and passing over the wall passes 

 over the upper guage discharged of rain, driving away an equal 

 volume of saturated air. We may also expect the one at the 

 base even at the windward side to receive less than others at a 

 distance from the wall, for part of the wind must be reverberated 

 over that. In short there will be a general mixture and confu- 

 sion of saturated and emptied volumes of air disturbing the 

 indications of all the guages within the influence of the eddies of 

 wind. 



Take another case. — Suppose an extended plain abruptly 

 broken by a perpendicular clifl'into two different levels. Say, a 

 clifF running from north to south, and on the west side of it a 

 high level plain, and on the east side a plain on a lower level. 

 Place a rain-guage on the edge of the cliff". Suppose a driving 

 rain with a west wind. This guage will receive its due quantity 

 of rain equally with others on the open plain* But suppose a 



