74 On the Productmi of Vapor. 



by evaporation. The water intended to be cooled is put 

 into very porous vessels, and exposed to the sun, or to a 

 current of warm air. In experiments made by Mr. 

 Richmami' in 1747, and inserted in the first volume of 

 the Transactions of the Imperial Academy of Petersburg, 

 a thermometer taken out of water and exposed to air of 

 equal temperature, descended and remained below the 

 height indicated by the water, until the bulb became dry, 

 when it regained its former height. If the ball of a ther- 

 mometer be wrapped in fine linen, and kept moist by 

 sprinkling with ether, and the evaporation be facilitated 

 by agitation in the air, the thermometer will descend to 

 O' Doct. Franklin has proved, that when the body per- 

 spires copiously, it is less heated than surrounding bod- 

 ies ; and that perspiration alway produces a certain de- 

 gree of coldness. A surprizing degree of cold is produ- 

 ced by a solution of the crystallized salts. By using a 

 saline mixture composed of eleven parts of dry pulveri- 

 zed Sal Ammoniac, ten parts of dry pulverized Nitre, 

 sixteen parts of Glauber's Salts, and thirty-two parts of 

 water, Mr. Walker has brought the thermometer to eight 

 degrees below 0. 



Another remarkable phenomenon attending the form- 

 ation and ascent of vapor is, that the less the gravity of 

 the air is^ the more copiously is the vapor exhaled, and the 

 greater its specific gravity. Chaptal observes, that evap- 

 oration is more speedy in proportion as the pressure of 

 the air is less upon the surface of the fluid. The Abbe 

 Rochon has applied this principle to distillation with sin- 

 gular advantage. It was found by the Abbe Mongez 

 and Mr. Lamanoiv, that ether evaporated with prodigious 

 facility upon the peak of Teneriffe, The same fact was 

 observed by Mr. Saussure on the mountains of Switzer- 

 land.^ Whilst Doct. Halley was making his observations 

 for a catalogue of the Southern stars, on the tops of the 

 mountains in the island of St, Helena, such an uncommon 

 quantity of vapor fell there in dew, as very much impe- 

 ded his business, by covering his glasses over in six or 

 seven minutes. In the account of an uncommon dark- 

 ness on May 19th, 1780, contained in the first volume of 

 the Memoirs of the American Academy of Arts and Sci- 



