444 The Account of a 
every thing which could be expected from them. Instead 
of the reflector being exposed to the wind, these lamps 
were inclosed in strong tin cases, having plates of ground 
glass in their fronts, which effectually prevented the bad ef- 
fects of an unequal and unsteady light. In the centre of the 
back of each case, there were straps and semicylinders of tin, 
which moving upon joints, clasped the staff to which in their 
use they were braced. Two of the lamps were of twelve 
inches diameter, and a third of twenty-two ; and the last of 
these, prior to the use of it in the ensuing season, was lighted 
on Shooter's Hill, and clearly distinguished at the distance of 
thirty miles. Copper nozles of different sizes were likewise 
provided for holding the white lights. 
During the measurement of the base, an observatory for 
the reception of the instrument was making at^ the Tower, as 
likewise two carriages, to be used in conveying them from sta- 
tion to station. One was made with springs for the greater 
safety of the instrument, which resting upon a cushion in the 
carriage, was sufficiently secured from any jolting upon the 
road. 
As it was easily foreseen that upon eminences, on which it 
was certain the instrument would be placed, it would be ha- 
zardous to trust it in a receptacle of slight construction, great 
pains had been taken to make the observatory strong. It con- 
sisted of two parts, the interior one of which, or the observa- 
tory itself, was eight feet in diameter, and its floor of a circu- 
lar form, and from the sides of it eight iron pillars rose to the 
height of seven feet, which were connected at the extremities 
by oaken braces. The roof was formed of eight rafters which 
limited at the top, having their ends fastened into the heads of 
