the Trigonometrical Operation . 1 6 3 
likewife two reverberatory lamps. Thefe two were attached 
to the fame iron bar, at the diftance of three feet from each 
other. They had concave copper reflectors, nine inches in dia- 
meter, extremely well polifhed and lilvered. They were in- 
tended at firft for experiments near London, and were very 
well feen at the diftance of fifteen or fixteen miles. To fee u.re 
us from any uncertainty that might have arifen, by miftaking 
other lights for our own, one lamp was placed over the other. 
But when we came afterwards to be better acquainted with the 
appearance of thefe lamps, that precaution was found to be 
entirely unneceffary ; wherefore fingle reverberatories were pro- 
vided, with Jpecula of ten inches diameter, and they were fup- 
plied with ftill larger burners, which could be feen at the dif- 
tance of twenty or twenty- four miles. But here it is proper to 
remark, that thefe lamps muft be carefully watched, efpecially 
in expofed windy fituations ; for if the cotton be drawn out a 
little too far, they are apt to fmoke, whereby the front glafs 
becomes obfeure, and therefore muft be wiped frequently. 
They are eafily turned on the pofts that fupport them ; and 
were, by the help of a telefcope laid on one fide, parallel to 
the axis of the rays (for which a contrivance was provided in the 
tin work) accurately prefen ted towards the ftation occupied by 
the inftrument at the time from whence they were to be ob- 
ferved. There was conftantly one of thefe lamps, and fome- 
times two, at two different ftations, burning each night, when 
we were making obfervations of the pole ftar, or white lights 
of fhort duration, placed at other diftant ftations. 
