DISCOVERIES IN THE CELESTIAL SPACES. 341 



precipitated water was determined by weight.* In addition 

 to the condensation-hygrometer, which, 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 Leonardo da 

 Vincif) the absorption-hygrometer, composed of substances 

 taken from the animal and vegetable kingdoms, made by San 

 tori (1625), Torricelli (1646), and Molineux. Catgut and the 

 spikes of grasses were employed almost simultaneously. In- 

 struments of this kind, which were based on the absorption by 

 organic substances of the aqueous vapor contained in the at- 

 mosphere, were furnished with indicators or pointers, and small 

 counter-weights, very similar in their construction to the hair 

 and whalebone hygrometers of Saussure and De Luc. The 

 instruments of the seventeenth century were, however, defi- 

 cient in the fixed points of dryness and humidity so necessary 

 to the comparison and comprehension of the results, and which 

 were at length determined by Regnault (setting aside the sus- 

 ceptibility acquired by time in the hygrometrical substances 

 employed). Pictet found the hair of a Guanche mummy 

 from Tenerifie, which was perhaps a thousand years old, suf- 

 ficiently susceptible in a Saussure's hygrometer.^ 



The electric process was recognized 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 al- 

 ready frequently spoken,^ and which appeared in the year 



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



t Venturi, Essai sur les Ouvrages Physico-mathimatiques de Leonard 

 de Vinci, 1797, p. 28. 



X Bibliotheque Universelle de Geneve, t. xxvii., 1824, p. 120. 



$ Gilbert, De Magnete, 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 : Magnetic^ ut ita dicam, vel electric^ 

 attrahere (vim illam electricam nobis placet appellare . . . .) (p. 52) ; 

 effluvia electrica, attractiones electricae. We do not find either the ab- 

 stract expression electricitas or the barbarous word magnetismus intro- 

 duced in the eighteenth century. On the derivation of rjTiEKTpov, " tho 

 attractor and tho attracting stone," from il^tg and lA/fCfv, already in 

 dicated in the Timseus of Plato, p. 80, c, and the probable transition 

 through a harder ITisKTpov, see Buttmann, Mythologus, bd. ii. (1829), 

 s. 357. Among the theoretical propositions put forward by Gilbert 

 (which are not always expressed with equal clearness), I give the fol- 

 lowing: "Cum duo sint corporura genera, quae " manifestis seusibui 



