June 11, 1915] 



SCIENCE 



873 



reference to wind action. When ripples or 

 waves are formed on a free water surface the 

 surface is very markedly altered, and this kind 

 of alteration occurs so generally and so un- 

 controllably with pan or tank atmometers ex- 

 posed in the open, that it renders futile any 

 detailed study of evaporating power carried 

 out with such instruments. Properly cared 

 for and properly operated, the porous cup sur- 

 face does not appreciably alter, at any rate it 

 is never altered by wind action. Other ad- 

 vantages of the porous cup over the free water 

 surface are related to errors in reading the 

 latter type of atmometer arising from splash- 

 ing and spray, the removal of water by animals 

 and the capture of animals, wind-blown leaves, 

 etc., in the tank; none of these errors are en- 

 countered in the operation of the Babinet 

 atmometer. Also, the porous cup may be so 

 mounted as not to absorb rain, which always 

 plays havoc with readings from open pans. 



For certain purposes, however, such as the 

 study of atmospheric evaporating power at or 

 near the soil surface or the surfaces of reser- 

 voirs, etc., the porous cylinder is not well 

 suited; here a plane evaporating surface is 

 frequently requisite. The well-known Piche 

 atmometer, or Cantoni's* or Houdaille's^ modi- 

 fication of the latter, all three employing 

 blotting-paper disks, may be used in such cases, 

 but these instruments are less convenient 

 in operation than is the porous cup, in vari- 

 ous ways. It therefore seemed desirable to 

 bring into use what amounts to a porous clay 

 cup with a plane evaporating surface. Just 

 such an instrument was devised and described 



*Cantoiii, G., "Sulle condizioni di forma e di 

 esposizione piu opportune per gli evaporimetri, ' ' 

 Send. S. 1st. Lomb., 11., 12 : 941-46, 1879. 



5 Houdaille, F., "Mesure de 1 'Evaporation 

 diume; description d'un 6vaporini6tre enregis- 

 treur," Bui. M4t. E4rault, 1890. (TMs is the in- 

 strximent catalogued by Richard Frgres, Paris. For 

 further references to Houdaille's work, as well as 

 that of others, see: Livingston, Grace J., "An An- 

 notated Bibliography of Evaporation," Mo. 

 Weather Sev., 36: 181-86, 301-06, 375-81, 1908; 

 37: 68-72, 103-09, 157-60, 193-99, 248-52, 1909. 

 Reprinted, repaged, Washington, 1909. This in- 

 cludes most of the papers appearing before 1908.) 



very long ago by BeUani,' who appears to 

 deserve credit for first employing a water-im- 

 pregnated solid connected by a simple water 

 column with a reservoir at a lower level. The 

 principle here involved forms the basis of the 

 Piche-Cantoni and Babinet instruments aa 

 afterwards constructed. 



It is hard to understand why Bellani's de- 

 vice should have remained unmentioned during 

 nearly a century of climatological and meteor- 

 ological advance. Especially surprising is this 

 lack of recognition when it is appreciated that 

 this horizontal plate of Bellani is exactly the 

 form of instrument that should have replaced 

 the open pan in the vain struggles of meteor- 

 ological students to obtain the long-sought 

 " evaporation formula " ; thus might have 

 been avoided, perhaps, some of the wasted 

 effort expended in attempts to express by a 

 single term the influence of wind upon evapo- 

 ration, when the latter was measured from the 

 variously rippled surface of free water. 



In Bellani's atmometer, a horizontal porous 

 clay disk closed the top of a vessel completely 

 filled with distilled water, so that the lower 

 surface of the disk was in contact with the 

 liquid, while the upper surface was exposed to 

 the air. A horizontal, graduated glass tube, of 

 small bore, open at its distal end, projected 

 laterally from the vessel, and the air-water 

 meniscus in the tube progressed toward the 

 vessel as evaporation occurred, the tube thus 

 becoming more or less rapidly emptied of 

 water. A suitable reservoir and cock allowed 

 the meniscus to be pushed back over the 

 scale, by admitting more water into the vessel. 

 As water evaporates from the porous surface 

 of such an instrument more is imbibed from 

 below, and air pressure drives the meniscus 

 inward along the scale, keeping the vessel com- 

 pletely filled and the plate continuously in 

 contact with the water below. The instrument 

 may be mounted with a burette reservoir, as is 

 frequent with the porous cups now in use, in- 

 stead of the horizontal graduated tube. 



After some preliminary discouragement I 

 have at length been able to obtain circular 



6 Bellani, A., "Descrizione di un nuovo atmido- 

 metro, etc.," Gior. Fis. Chim., 311: 166-77, 1820. 

 (Eeprinted, Pavia, 1820.) 



