424 Mr. W. S. Jevons on the Deficiency of Rain in an 



supposed fixed in mid-air. It is here, I venture to hope, ren- 

 dered quite plain that less rain will fall upon the summit of the 

 obstacle than elsewhere, the surplus being carried forward to the 

 lee side of the obstacle. I entertain no doubt that we have in 

 this process a sufficient explanation of the observed deficiency of 

 rain in elevated places. 



8. It is an evident corollary of this explanation, that no defi- 

 ciency of rain would be observed did the measuring instrument 

 cause no disturbance in the wind. But only a gauge of which 

 the month is level with the ground fulfils this condition. Pro- 

 bably, indeed, the church-tower or house upon which a gauge is 

 usually elevated occasions the chief part of the deficiency. Hence 

 a gauge suspended in mid-air would collect more rain than if it 

 were placed on a house. Yet a rain-gauge is itself an obstacle 

 of some importance, and will cause a part of the rain to pas3 on 

 unmeasured, as I have attempted to represent in fig. 3. The 

 hollow of the funnel in this figure, it will be observed, is filled 

 up with an eddy of wind. 



9. In these drawings, I should observe, some little exaggera- 

 tion must be excused : no notice, too, is taken of the motion of 

 the wind in the third dimension of space, that is, round the 

 obstacle instead of over it. 



10. Thus having reason to suppose that the deficiency of rain 

 at elevated points was due to the disturbance of the wind, I have 

 examined all the observations and statements I could meet with 

 bearing on the subject, and find my opinion, on the whole, 

 strongly confirmed. 



An intelligent observer, Mr. H. Boase of Penzance, after 

 four months' experiment, remarks*, " Having observed that 

 the difference between the first and the other gauges varied with 

 more or less wind, its velocity has been registered from observa- 

 tion; but not having an accurate anemometer, we cannot yet 

 offer any certain conclusion further than this, that the difference 

 of the quantity of rain received in a gauge placed on the top of 

 a building, and one at a level with the surface of the ground, is, 

 for some reason or other, proportional to the velocity of the 

 wind." 



11. Again, taking the measurements of rain t made by Luke 

 Howard, and arranging them in the order of the ratio of the 

 quantities in the lower and higher gauges, we find that we have 

 also arranged them almost exactly in the order of the amount 

 of accompanying wind, as indicated by the notes annexed. The 

 results are as follows : — 



* Annals of Philosophy (July 1822), new scries, vol. iv. p. 18. 

 t Howard's 'Climate of London,' vol. ii. p. 158. 



