213 



Observations on the rate of Evaporation on the Open Sea ; with a de- 

 scription of an Instrument used for indicating its amount. r By T. 

 W. Laidley, Esq. 



It has often occurred to me, that a simple and convenient instru- 

 ment for ascertaining the actual amount of exhalation from a humid 

 surface, could not fail of being essentially serviceable to meteorologi- 

 cal science, as well as to the arts. An instrument for this purpose 

 was indeed contrived by the late Professor Leslie, to which he gave 

 the name Atmometer : but though very ingenious, and fulfilling 

 tolerably well the intentions of the inventor, it fails in a very impor- 

 tant qualification of scientific instruments, simplicity of construction 

 and use ; and is consequently less frequently employed in observing 

 the condition of the atmosphere in reference to dryness and humidity 

 than is desirable. The instrument is thus described by its inventor • 

 " The Atmometer consists of a thin ball of porous earthenware, two or 

 three inches in diameter, with a small neck, to which is firmly 

 cemented a long and rather wide glass tube, bearing divisions, each of 

 them corresponding to an internal annular section, equal to a film of 

 liquid that would cover the outer surface of the ball to the thickness 

 of the thousandth part of an inch. The divisions are marked by por- 

 tions of quicksilver introduced, ascertained by a simple calculation, 

 and they are numbered downwards to the extent of 100 to 200; to 

 the top of the tube is fitted a brass cap, having a collar of leather, 

 and which after the cavity has been filled with distilled water, is 

 screwed tight. The outside of the ball being now wiped dry, the in- 

 strument is suspended out of doors, exposed to the free access of the 

 air. In this state of action the humidity transudes through the 

 porous substance just as fast as it evaporates from the external sur- 

 face ; and this waste is measured by the corresponding descent of 

 water in the stem. If the Atmometer had its ball perfectly screened 

 from the agitation of the wind, its indications would be proportional 

 to the dryness of the air at the lowered temperature of the humid 

 surface ; and the quantity of evaporation every hour as expressed in 

 thousand parts of an inch, would when multiplied by 20 give the hy- 

 grometric measure. The Atmometer is an instrument evidently of 

 extensive application, and of great utility in practice. To ascertain 

 with accuracy aud readiness the quantity of evaporation from any 



