Mr. T. Tate on a new Evaporameter and Absorbometer. 131 



water. By means of this instrument the following laws of 

 evaporation have been established : — 



1. Other things being the same, the rate of evaporation is 

 nearly proportional to the difference of the temperatures indi- 

 cated by the wet- and dry-bulb thermometers. 



2. Other things being the same, the augmentation of evapo- 

 ration, due to air in motion, is nearly proportional to the velocity 

 of the wind. 



3. Other things being the same, the evaporation is nearly 

 inversely proportional to the pressure of the atmosphere. 



By means of Apjohn's formula, or by Glaisher's multipliers 

 and a special constant, this instrument therefore may be used 

 as a hygrometer, giving indications vastly more delicate, if not 

 more reliable, than those of the wet- and dry-bulb thermometer 

 commonly employed. It is believed that the proposed instru- 

 ment would supply a desideratum in meteorological observations, 

 inasmuch as it would give with the utmost delicacy the evapo- 

 rating capacity of the atmosphere, or, what amounts to the same 

 thing, the comparative dryness and salubrity of the atmosphere 

 extending over given intervals of time; whereas other hygro- 

 meters only give the state of the atmosphere at the particular 

 time of observation. When used as a hygrometer, the instru- 

 ment should be placed in a situation where there are no currents 

 of air. 



Results of Experiments relating to Absorption. 



The following experimental researches form a continuation of 

 those given by me in the Philosophical Magazine for 1860-61. 



1. When water is diffused from a central point through the 

 pores of an absorbent (with a certain proviso), nearly equal 

 volumes of water are absorbed in equal times. 



Dry calico, in four folds, was laid on the tube of the absorbo- 

 meter represented in fig. 2. At first the absorption was very 

 rapid, but after the lapse of a few minutes it became nearly uni- 

 form ; thus the number of minutes corresponding to each suc- 

 cessive interval of 2 inches on the tube were 3*16, 3*3, 3*5, 

 3-6, 3-8, 38. 



Plaster of Paris being placed on the tube, gave the following 

 results : the numbers of seconds corresponding to successive 

 intervals of 4 inches on the tube were 55, 60, 62, 64. 



For a very porous sandstone, the numbers of seconds corre- 

 sponding to successive intervals of 4 inches on the tube were 

 70, 75, 80, 80. 



For a sandstone of greater density and closer in its pores, the 

 numbers of minutes corresponding to the absorption of succes- 

 sive half cubic inches of water were 36, 39, 41 . Similar results 



K2 



