74 Meteorology . 



variation of height, — as between the top and bottom of a steeple 

 or house, — is materially different. In the year 1767 some experi- 

 ments were made at Westminster. Two similar gauges were 

 placed one at the top, the other at the bottom of the tower of the 

 Abbey, and it was found in the course of twelve months that the 

 highest gauge received or caught only twelve inches of rain; while 

 the one placed on the ground caught nearly double ; viz. twenty- 

 two inches and a half. Other experiments were made in different 

 places, with the same general result ; which shows that the height 

 the gauge is placed from the ground must be noted before com- 

 parative results can be deduced. We are in want of the ratio of 

 quantity caught at any stated height when compared with that 

 which may have fallen on the surface of the earth beneath. One 

 step has been made by Mr. Dalton, who has shown that a gauge 

 elevated 150 feet from the ground, caught in winter 07ie half, and 

 in summer two-thirds as mucli as a gauge placed on the ground 

 below. Yet still the general ratio of height and quantity is 

 w-anting, and the ascertaining this question will be a valuable ac- 

 quisition to meteorological science. 



While this research is in progress, it may be agreeable to your 

 readers to have a periodical statement of the quantity of rain 

 that falls, and at the same time the quantity of evaporation that 

 takes place from the surface of water. 



The apparatus with which these observations are made, con- 

 sists of a rain-gauge or ring (such as used by the Royal Society) 

 of twelve inches in diameter ; the water caught in this area is 

 measured in laminae of the one-thousandth of an inch thick, and 

 the area of the ring. 



The evaporating instrument consists of a vessel of the same 

 area as the rain-gauge, viz. twelve inches diameter and five inches 

 deep, and is placed at the same height from the ground, and 

 consequently receives an equal quantitvof water a,s the rain-gauge. 

 In the evaporator a known quantity of water is put, and the 

 vessel is left open to the action of the air and sun ; the surface 

 of the water being protected from the depredation of birds, &c. 

 by the intervention of a net of large meshes composed of fine 

 wire. The registry of quantity of rain and evaporation is made 

 weekly on the same day and hour ; and the apparatus is fixed so 

 that the upper edges or gauge-rings are four feet from the ground. 



I shall now proceed to give you the observations made since 

 .January 1817, and they will I trust prove sufficiently curious to 

 attract notice. I am, dear sir, yours, 

 Croydon, Jan. 1818. H. LawsoN, 



[In the subjoined Table the heights are expressed in inches and 

 decimal parts.] 



Months. 



