DISCOVERIES IN THE CELESTIAL SPACES. 725 



vapour by evaporation and precipitation. The oldest Flo. 

 rentiiie hygrometer was accordingly a condensation-hygro- 

 meter, an apparatus in which the quantity of the discharged 

 precipitated water was determined by weight.* In addition 

 to the condensation-hygrometer, w r hich, by the aid of the 

 ideas of Le Roy in our own times, has gradually led to the 

 exact psychrometrical methods of Dalton, Daniell, and August, 

 we have (in accordance with the examples set by Leo- 

 nardo da Vinci f), the absorption-hygrometer, composed of 

 substances taken from the animal and vegetable kingdoms, 

 made by Santori (1625), Torricelli (1646), and Molineux. Cat- 

 gut arid the spikes of grasses were employed almost simul- 

 taneously. Instruments of this kind, which were based on the 

 absorption by organic substances of the aqueous vapour con- 

 tained in the atmosphere, were furnished with indicators or 

 pointers, and small counter-weights, very similar in their 

 construction to the hair and whalebone hygrometers of Saus- 

 sure and Deluc. The instruments of the seventeenth century 

 were, however, deficient in the fixed points of dryness and 

 humidity so necessary to the comparison and comprehension 

 of the results, and which were at length determined bv Reg- 

 nault ; (setting aside the susceptibility acquired by time in the 

 hygrometrical substances employed.) Pictet found the hair of 

 a Guanche mummy from Teneriffe, which was perhaps a thou- 

 sand years old, sufficiently susceptible in a Saussure's hygro- 

 meter .j 



The electric process was recognised by William Gilbert as 

 the action of a proper natural force allied to the magnetic 

 force. The book in which this view is first expressed, and in 

 which the words electric force, electric emanations, and elec- 

 tric attraction are first used, is the work of which I have 

 already frequently spoken, and which appeared in the year 



* Antinori, p. 45, and even in the Saggi, p. 17-19. 



f Venturi, Essai sur lea ouvragcs physico-mathcmatique* de 7eo- 

 nard de Vinci, 1797, p. 28. 



I Bibliotheque universellede Geneve, T sxvii. 1824, p. 120. 



Gilbert, de Magnate, lib. ii. cap. 2-4, p. 46-71. With respect to 

 the interpretation of the nomenclature employed, he already said : 

 Electrica quae attrahit eadem ratione ut electrum ; versorium non mag- 

 neticum ex quovis metallo, inserviens electricis experimentis. In the 

 text itself we find as follows: Mignetice ut ita dicam, vel electric* 

 ftttrahere (vim illam electricam noto placet appellare . . . .) (p. 52) 



