I. MEASUREMENT. 397 



There are garnets in the bed plates of the supports so as to lessen the 

 friction. For turning the telescope there is a ring of gutta-percha. The 

 telescope carries a level, which can remain permanently on it ; one division of 

 which = 1 /A 43. It has also an altitude arc graduated to 10' and reading to 

 1', by means of a vernier. The Nadir distance can be found by an artificial 

 horizon. One division of the micrometer head = y^gth of a revolution, and 

 the pitch of the screw is about y^th of a Paris inch. 



By this instrument can be determined 



1 . The time on the meridian. 



2. The time in the vertical of the Polestar. 



3. When it is turned through 90', the latitude, according to the method 



ofBessel. 



4. The latitude, according to Talcott's method, where the difference 



between the north and south zenith distance is measured by the 

 micrometer screw. 



1771. Portable Catoptric Transit Instrument, with a 

 telescope horizontally resting in collars, and revolving only on its 

 own optic axis. Invented and constructed by C. A. Steinheil, 

 sen 1 '. 



Conservatorium of the Math, and Phys. Collections of 

 Bavaria (Prof. Dr. Scidcl). 



The construction of this instrument is described by its inventor in Schu- 

 machers Jahrbuch fur 18J4 (Stuttgart and Tubingen, published by Cotta), 

 p. 3, et seq. 



177 la. Reflecting Transit Instrument. 



W. Watson and Son. 



In this the image of the star is received on a miiTor, and then viewed by 

 looking down through a small telescope placed in such a position as to be most 

 convenient for observation ; the telescope is stationary, but the mirror moving 

 in the plane of the meridian, can be directed to any point in it, and obser- 

 vations taken without those inconvenient positions of the observer, so often 

 necessary with transits of ordinary construction. 



1775a. Arabic Quadrant in bronze, made by Ahmed Ibu 

 Abd el Rahman. Archceological Museum, Madrid. 



Its radius is 0'21 1U . It has an inscription in Ensic letters of the latter end 

 of the middle ages giving the name of the maker. 



" Quadrant Orario " of Cosimo I. dei Medici, to the latitude 



of 43 45', made in Florence in 1566, probably bylgnazio Danti. 



The Royal Institute of " Studii Superiori" Florence. 



Interesting likewise, because it contains calculations relative to the reform 

 of the calendar, afterwards carried out by Pope Gregory XIII. 



" Quadrant Orario " to the latitude of 43 45', made in 

 Florence in 1565 by Giovan. Battista Giusta. 



The Royal Institute of" Studii Superiori" Florence. 



" Instrumento del Primo Mobile " of Apiano, exe- 

 cuted in Florence in 1568 by Ignazio Danti. 



The Royal Institute of il Studii Superiori" Florence. 



