114 M. Flaugergues on the Quantity of Rain, and [Aug. 



To find the cause of this difference, it is sufficient to consider 

 that the air is seldom perfectly calm during rain. A wind of 

 more or less violence usually blows, which, pushing horizontally 

 the drops of rain in their vertical fall, gives them a direction 

 inclined to the horizon. Hence it follows that less rain will fall 

 into the horizontal opening of the rain gauge when the rain is 

 inclined than if it fell vertically or in a direction less inclined. 



Let us suppose that the parallel and inclined lines (fig. G) 

 represent the direction in which the drops of rain move in a 

 vertical plane, and that the line A B is a section of the rain 

 guage in the same place. Let us draw A D perpendicular to 

 the direction of the rain, and equal to A B ; it is evident that all 

 the rain which enters into the rain guage by A B, in consequence 

 of its inclined direction, may be intercepted by the part A C of 

 the line A D which receives it perpendicularly ; but if the rain 

 fell vertically into the rain-guage, the part A B would receive as 

 much as the line A D receives in the present case ; therefore, 

 the quantity of rain which enters into the rain guage when its 

 direction is inclined is to the quantity which would enter into 

 the same vessel if the rain fell vertically as A C to A D (= A B), 

 or in consequence of the right angled triangle, A C B, as the 

 sine of the angle, ABC, to radius ; that is to say, that the 

 quantity of rain which enters into the rain guage is proportional 

 to the sine of the angle of inclination of the rain.* 



The rain at the top of the house, or any other part completely 

 exposed, experiencing without obstruction the whole action of 

 the wind, must assume a direction inclined to the horizon, and 

 reach the rain guage placed there in that direction ; but if the 

 rain is screened from the action of the wind by the building, the 

 drops gradually losing their inclined direction by the resistance 

 of the air must fall into the rain guage placed there vertically, or 

 nearly so. Hence more rain must enter a rain guage placed at 

 the bottom of a building than into one placed on the parapet, as 

 has been observed by the acute philosophers above named. 



This remark may give origin to the question, where the rain 

 guage for meteorological purposes should be placed. I answer, 

 that the solution of the question depends upon the object in view. 

 If we wish to know the quantity of rain which falls upon a 



• I employed myself formerly in measuring the inclination of rain, at least the 

 greatest inclination which took place during a storm, &c. The cliseometer, which 

 I contrived for this purpose, consisted in a very thin circular plate of metal, kept in 

 a situation exactly horizontal by a long cylindrical support of wood, the axis of 

 which passed through the centre of the plate, and which was fixed vertically and 

 colidly on a small stone pedestal in a place quite exposed. The wooden support 

 was painted red, which became much deeper when moistened. After the rain, I 

 measured the length of the part of the wooden support which remained dry, and I 

 calculated this proportion : the difference of the semidiameters of the metallic 

 plate and the wooden support is to the length of the part of the support which 

 remained dry as radius is to the tangent of the angle of inclination of the rain. 

 Further, by the circular extent ard the disposition of the moistened part along th« 

 support, 1 could discover the winds which had blown during the storm. 





