326 IJeut. Drummond on the means of facilitating the 
allude to Leith Hill, near Dorking, in Surrey, and Wrotham 
Hill, in Kent, stations which it was of the utmost conse- 
quence to observe from Berkhampstead Tower, near Hert- 
ford. Our efforts to effect these observations having for 
some time been rendered unavailing by the thick mist, so 
frequently overhanging the bed of the Thames, a series of 
bright tin plates was put up on both stations. Each set con- 
sisting of six or eight plates, was attached to a smooth flat 
board, placed vertically by the plumb line, and turning on a 
pivot: the respective inclinations of the plates with the face 
of the board being determined, so that they might have the 
positions required for reflecting, in succession, the sun's rays 
towards Berkhampstead tower, when the surface of the board 
was turned at right angles to the line of direction. Although 
this method admitted but of rude execution, it fully answered 
the purpose for which it was employed : the plates became 
visible in succession at the appointed hours, the duration of 
each varying with the inequality of its surface, but being 
generally from ten to fifteen minutes ; they were seen nearly 
at the same hours for some days before and after that for 
which they were calculated. 
The distance to Leith Hill is 45 miles, and the observations 
were in this way completed without the hill itself having been 
visible during the whole of our stay, which was nearly three 
weeks. 
The utility of employing the sun's reflection as a point 
of observation being established by the result of these 
experiments, it only remained, instead of a temporary 
expedient, rather difficult of execution, to substitute an 
instrument that might be used on all occasions, simple 
