vi Proceedings of Boyal Society of Edinburgh. 
tioD. In one respect the altazimuth marked a new departure in the 
construction of astronomical instruments. Instead of being built up 
of many parts hound together by numerous screws, it was made in 
as few pieces as possible, and the greatest care was taken that the 
critical parts, such as the micrometer screws, bore immediately against 
solid portions of the mounting. With this instrument the moon 
was closely followed through its smaller phases for many years, and 
if the resulting places have not fulfilled the expectations raised by 
them, it is chiefly because of the difficulty of obtaining a very exact 
record from any instrument free to move in two co-ordinates, and 
partly also from the somewhat insufficient optical power of the tele- 
scope employed. The altazimuth was, however, a hold experi- 
ment, and cleared the way for a new instrument about to be intro- 
duced at Greenwich, viz., the Universal Transit-Circle, which seems 
destined to accomplish in the most satisfactory manner the task 
originally proposed by Airy. In 1848 the late Astronomer Eoyal 
replaced the old zenith-sector by the reflex zenith-tube, with which 
y Draconis, the star which passes almost exactly overhead at Green- 
wich, is observed from time to time. Greenwich is indeed favoured 
by the position of this star, so near at once to the zenith, and to the 
solstitial colure, that for many centuries it will continue to culminate 
within a few minutes of the zenith, just as it did in the days of 
Hooke, by whom it was first observed with extreme accuracy. 
In 1850 the large and massive transit-circle built, like almost all 
the Greenwich instruments, by Troughton and Simms, superseded 
the transit instrument and mural circles till then in use. It is still 
a magnificent instrument, and one of the most stable of its class. 
Like the altazimuth it is almost altogether made of cast-iron, but 
unlike the greater number of transit-circles it cannot be reversed ; 
this disadvantage is, however, very possibly outweighed in a national 
observatory by the unbroken nature of the records obtained. The 
chronograph was introduced at Greenwich some four years after 
the transit-circle. 
In 1859 the Greenwich plant was augmented by a large equatorial 
of 12f inches aperture. The object-glass was made by Merz, but 
the mounting, which resembles that of the Northumberland tele- 
scope at Cambridge, was designed by Airy. So satisfactory and 
steady has this mounting proved that it is now being provided with 
