observation of distant stations in geodcetical operations. 333 
two inches in diameter blown at one extremity, were filled 
with a mixture of these gases, and placed within two inches of 
the luminous ball of lime, which was J- of an inch in diameter. 
The additional light, reflected by a concave mirror 9% inches 
in diameter, was also concentrated on one of the balls. 
After some minutes exposure to the light the glass began 
to get dim, especially on the sides nearest the light, being 
covered with a thin whitish film ; at the end of 15 minutes 
they were withdrawn, and the stop-cocks being opened, 
under water coloured with an infusion of litmus, it imme- 
diately rushed in, filling one-third of the ball on which the 
reflected light had also been concentrated, and one-half of the 
other exposed to the direct light only : the difference pro- 
bably arising from some accidental inequality in the bright- 
ness of the opposite sides of the ball. The purple tint of the 
litmus was rapidly changed to red, which, again by the ac- 
tion of the uncombined portion of chlorine, gradually faded 
away, and was soon destroyed. The effect of the light on 
chloride of silver was equally remarkable. Three portions, 
one at the distance of about an inch from the luminous ball, 
the other four, and the third in the focus of the violet rays, 
concentrated by a powerful lens, were exposed to the influ- 
ence of the light for twelve minutes, and were then found to 
be completely discoloured ; that nearest the light being almost 
black, the other two having a brownish black hue. Having 
now described the instruments contrived to facilitate the 
observation of the distant stations of the survey, it may not 
perhaps be uninteresting to add an account of their success- 
ful application to a case of considerable importance and dif- 
ficulty, that occurred at the end of last season. 
X x 
MDCCCXXVI. 
