THE UNIVERSE. DISCOVERIES IN THE CELESTIAL SPACES. 339 



precipitation. The oldest Florentine hygrometer was accord 

 ingly a condensation hygrometer, an apparatus in which the 

 quantity of precipitated water which ran off was determined 

 by its weight. ( 525 ) To this condensation hygrometer, which, 

 aided by the ideas of Le Roy, has gradually led in our own 

 days to the exact psychrometric methods of Dalton, Daniell, 

 and Auguste, there were added, according to the example 

 previously set by Leonardo da Vinci, ( 526 ) the absorption 

 hygrometers made of animal or vegetable substances, of 

 Santori (1625), Torricelli (1626), and Molineux. Catgut, 

 and the beards of a wild oat, were used almost at thr same 

 time. Instruments of this kind, founded on the absorption 

 of the aqueous vapour contained in the atmosphere by organic 

 substances, were provided with indexes and counterpoises, 

 and were very similar in construction to Sa\issure's and 

 Deluc's hair and whalebone hygrometers; but the instru- 

 ments of the 17th century were deficient in the determina- 

 tion of fixed dry and wet points, so necessary for the 

 comparison and understanding of the results. This desi- 

 deratum was at last supplied by Regnault, but without 

 reference to the variation which might be occasioned by time 

 in the susceptibility of the hygrometric substances employed. 

 Pictet, ( 527 ) however, found that the hair of a Guanche 

 mummy from Teneriffe, which might be a thousand years 

 old, employed in a Saussure's hygrometer, still possessed a 

 satisfactory degree of sensibility. 



Electric action was recognised by William Gilbert as 

 the operation of a natural force or power allied to magnetism. 

 The book in which this view was first enounced, and even 

 in which the terms " electric force," " electric emanations/' 



