observation of distant stations in geodcetical operations. 335 
experienced in observing Slieve Snaght appears to have 
arisen not so much from the distance, as from the direction 
passing over a range of other hills, and not far above their 
tops. Of twelve observations, made during the day, on the 
ninth and tenth of November, not more than one could have 
been effected without the aid of the heliostat, and many 
were obtained when the outline of the hill itself had ceased 
to be visible. The light at night was referred to a 15 
inch parabolic reflector, illuminated by an argand lamp, 
and placed on the church tower of Randalstown, nearly in the 
same direction. The light on the distant hill was not only 
seen with the naked eye, but appeared much brighter and 
larger than that at Randalstown, their relative distances being 
nearly 67 and 15 miles. Colonel Colby proposes employing 
this light to effect the observation of Ben Lomond from 
Knock Layd, in the north-east extremity of Ireland, a dis- 
tance of not less than 95 miles ; and availing himself of the 
position of Ben Lomond, which commands a view of the Ob- 
servatory on the Calton Hill of Edinburgh, to determine by 
simultaneous observations, the difference of longitude between 
it and Knock Layd, which is nearly in the meridian of the 
Dublin Observatory. 
It may perhaps be asked, if this mode of producing light 
does not admit of being applied to other purposes, besides 
those of a Trignometrical Survey : and, that, which naturally 
suggests itself, as a very extensive and important application;, 
is the illumination of light-houses. Not being subject to ex- 
plosion, it might be employed without incurring the slightest 
danger : and the apparatus, when fixed and permanent, might 
be rendered so simple, that the most ordinary care and attention 
