ATMOMETRY AND THE ATMOMETER 57 



The Bellani Porous Plate (Fig. 4) 



This kind of evaporating surface, 10 which appears never to 

 have been taken seriously during nearly a century that has 

 elapsed since Bellani described it, seems to possess a number 

 of advantages over the free water surface. A horizontal porous 

 clay disk closes the top of a vessel completely filled with dis- 

 tilled water, so that the lower surface of the disk is in con- 

 tact with the liquid while the upper surface is exposed to the air. 

 A small-bored, horizontal, graduated glass tube, open at its 

 distal end, projects laterally from the vessel, and as evaporation 

 takes place the air-water meniscus in this tube progresses toward 

 the vessel, the tube being thus more or less slowly emptied of 

 water. A suitable reservoir and controlling cock allows the 

 meniscus to be pushed back over the scale, by admitting more 

 water into the vessel. This is the earliest form of atmometer 

 with an imbibed solid evaporating surface and the water reser- 

 voir at a lower level. The possibility of such an arrangement 

 results from the fact that it requires considerable gas pressure to 

 force a passage of gas through such a plate, of porous pottery 

 when saturated with water. Thus, atmospheric pressure is 

 without influence upon the water mass in the vessel, excepting 

 upon the index meniscus. As water evaporates from the porous 

 surface more is imbibed from below and air pressure drives the 

 meniscus inward along the scale, keeping the vessel filled and the 

 plate continuously in contact with the water. It is to be noted 

 that the Piche-Cantoni paper disk and the Livingston paper 

 cylinder employ this same principle, only in those cases evapora- 

 tion takes place from the marginal part of the paper project- 

 ing beyond the end of the supply tube. The Piche-Cantoni 

 form is not unlike what we should have if we suppose Bellani's 

 porous plate to extend laterally much beyond the vessel which it 

 closes, at the same time supposing the diameter of this vessel to 

 be decreased until it becomes a mere supply tube from the lower 



10 Bellani, A., Descrizione di un nuovo atmidometro per servire di contin- 

 uazione e fine alle reflessioni critiche intorno all' evaporazione. Gior. Fis. Chim. 

 3 U : 166-177, 1820. Also reprinted; Pavia, 1820. 



