DISCOVERIES IN THE CELESTIAL SPACES. 705 



tern," the perceptible evidence of the translatatory motion of 

 the earth. 



Galileo had also early perceived (Septemoer 1612), the 

 importance of the occultations of Jupiter's satellites for geo- 

 graphical determinations of longitude on land. He pro- 

 posed this method, first to the Spanish court in 1616, and 

 afterwards to the States General of Holland, with a view of 

 its being applied to nautical purposes,* little aware, as it 

 would appear, of the insuperable difficulties presented to its 

 practical application on the unstable element. He wished to 

 go himself, or to send his son Vicenzio, to Spain, with a hun- 

 dred telescopes, which he would prepare. He required as a 

 recompense " una croce di San Jago," and an annual payment 

 of 4000 scudi, a small sum, he says, considering that hopes 

 had been given to him, in the house of Cardinal Borgia, of 

 receiving 6000 ducats annually. 



The discovery of the secondary planets of Jupiter was soon 

 followed by the observations of the so-called triple form 

 of Saturn as a planeta tergeminus. As early as November 



* Galilei, Opere, t. ii. (Longitudine per via de' Pianeti Medicei) 

 pp. 435-506; Nelli, Vita vol. ii. pp. 656-688; Venturi, Memorie e 

 Lettere di G. Galilei, P. i. p. 177. As early as 1612, or scarcely two 

 years after the discovery of Jupiter's satellites, Galileo boasted, some- 

 what prematurely indeed, of having completed tables of those secondary 

 satellites "to within 1' of time." A long diplomatic correspondence was 

 carried on with the Spanish ambassador in 1616, and with the Dutch 

 ambassador in 1636, but without leading to the desired object. The 

 telescopes were to magnify from forty to fifty times. In order more 

 easily to find the satellites when the ship is in motion, and (as he 

 believed) to keep them in the field, he invented, in 1617 (^elli, vol. ii. 

 p. 663), the binocular telescope, which has generally been ascribed to 

 the Capucine monk, Schyrleus de Rheita, who had much experience in 

 optical matters, and who endeavoured to construct telescopes magnify- 

 ing four thousand times. Galileo made experiments with his binocular 

 (which he also called a celatone or testiera), in the harbour of Leghorn, 

 while the ship was violently moved by a strong wind. He also caused a 

 contrivance to be prepared in the arsenal at Pisa, by which the observer 

 of the satellites might be protected from all motion, by seating himself 

 in a kind of boat, floating in another boat filled ^ith water or with oil 

 (Lettera al Picchena de' 22 Marzo, 1617; Nelli, Vita, vol. i. p. 281; 

 Galilei, Opere, t. ii. p. 473 ; Lettera a Lorenzo Realio del 5 Giugno, 

 1637). The proof which Galileo (Opere, t. ii. p. 454) brought forward of 

 the advantage to the naval service of hia method over M:rin'a method of 

 lunar distances is very striking. 



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