660 



PHILOSOPHICAL TRANSACTIONS. 



[anno 1769. 



ground of the garden belonging to the same house, and there was found the 

 same difference between these two, though placed so near each other, which 

 there had been between them, when placed at similar heights in different parts 

 of the town. After this fact was sufficiently ascertained, it was thought proper 

 to try, whether the difference would be greater at a much greater height ; and a 

 rain-gage was therefore placed on the square part of the roof of Westminster 

 Abbey, being at such a distance from the western towers, as probably to be very 

 little affected by them, and being much higher than any other neighbouring 

 buildings. Here the quantity of rain was observed for a twelvemonth, the rain 

 being measured at the end of every month, and care being taken, that none 

 should evaporate, by passing a very long tube of the funnel into a bottle through 

 a cork, to which it was exactly fitted. The tube went down very near to the 

 bottom of the bottle, and therefore the rain, which fell into it, would soon rise 

 above the end of the tube, so that the water was no where open to the air except 

 for the small space of the area of the tube: and by trial it was found, that there 

 was no sensible evaporation through the tube thus fitted up. The following 

 table shows the result of these observations. 



From July 7j I766, to July 7, 1767, there fell into a rain-gage fixed 



1766 from July 7 to the end 



August 



September 



October 



November 



December 



1767 January 

 Februar}' 

 March 

 April 

 May 

 June 



from July I to the 7 th 



Below the top 

 of a house, 

 inch. 

 3.591 

 0.558 

 0.421 

 2Mi 

 1.079 

 1.612 

 2.071 

 2.864 

 1.807 

 1.437 

 2.432 

 1.977 

 0.395 



22.6O8 



On the top 

 a house, 

 inch. 

 3.210 



0.479 

 0.344 

 2.061 

 0.842 

 1.258 

 1.455 

 2.491- 

 1.303 

 1.213 

 1.745 

 1.426 

 0.309 



18.139 



of 



On Westmin- 

 ster Abbey, 

 inch. 

 2.311 



} 



} 



0.508 



1.416 

 0.632 

 0.994 

 1.035 

 1.335 

 0.587 

 0.994 

 1.142 



1.145 



12.099 



By this table it appears, that there fell below the top of a house above a 5th 

 part more rain than what fell in the same space above the top of the same house, 

 and that there fell on Westminster Abbey not much above one-half of what was 

 found to fall in the same space below the tops of the houses. This experiment 

 has been repeated in other places with the same event. What may be the 

 cause of this extraordinary difference has not yet been discovered; but it may be 

 useful to give notice of it, in order to prevent that error, which would fre- 

 quently be committed in comparing the rain of two places without attending to 

 this circumstance. 



