ington astronomers emphasized Alvan Clark's competence, and ambi- 

 tion, to make the largest refracting telescope in the world 234 — that is, 

 one larger, if only by half an inch, than the 25-inch equatorial recently 

 built by Thomas Cooke for R. S. Newall in England. 235 



In America at that time there were neither glassmakers competent to 

 cast the large discs nor other opticians on a par with the Clarks. Pre- 

 paratory to beginning work on the Washington telescope, therefore, the 

 Clarks made several European business trips. In September 1870 George 

 went to Birmingham to engage Chance & Co. to cast the discs, and to 

 Gateshead to see Newall's telescope. Four months later, after observing 

 the solar eclipse from Spain, Alvan Graham went to England to examine 

 Newall's instrument for himself and to see how Chance was progressing. 

 In the fall of 1871 Alvan Graham returned to Birmingham to inspect 

 the discs before shipment, and to the continent to study other large 

 telescopes. 



The discs were figured and tested by the fall of 1872; after a slight 

 correction they were pronounced "very nearly perfect." The mounting 

 was designed by the Clarks in collaboration with Simon Newcomb. Like 

 the Newall telescope, the original Washington tube was made of sheet 

 steel rather than wood. The driving clock was invented by Newcomb 

 specifically for this instrument : it was driven by weights raised by water 

 from the city pipes; and it was regulated by a conical pendulum whose 

 isochronism was insured by an electromagnetically controlled friction 

 pad. The Clarks were also responsible for several of the accessory instru- 

 ments used with the great refractor : a two prism spectroscope, a chrono- 

 graph regulated by a conical pendulum, a variety of eyepieces, and two 

 micrometers. 236 The first micrometer was used until 1900, at which time 

 the second, with the addition of an electric illumination, was installed. 

 In his annual report for that year the superintendent of the observatory 

 noted that this "new" micrometer, made by George Clark in 1874, "rep- 

 resents the finest workmanship of a gifted mechanic." 237 



234 Astronomical and Meteorological Observations Made at the United States Naval Observa- 

 tory During the Tear i86g, p. v. 



235 Alvan Clark & Sons to Simon Newcomb, 17 June 1870 (letter in Newcomb 

 Papers at the Library of Congress) . 



236 Instruments and Publications of the United States Naval Observatory (Washington, 

 D.C., 1876), pp. 26-45. 



237 Report of the Superintendent of the United States Naval Observatory for the Fiscal Tear 

 Ending June 30,1 goo, p. 22. 



102 



