394 LIEUT. DRUMMOND ON THE ILLUMINATION OF LIGHT-HOUSES. 
Wharf, Blackwall ; but I have little to add to what I told you respecting those 
on the evening of the 25th instant : indeed it is not within the compass of lan- 
guage to describe accurately the details of such experiments, for it is by ocular 
evidence alone that their merits can be understood. 
“ Essentially the experiments of last evening were the same as those of the 
25th, and their effects likewise. The degrees of darkness in the evenings how- 
ever were so different, that some particular results were not the same. The 
moon last night, being nine or ten days old, lighted up the clouds so much, 
that even when the moon herself was hid, there was light enough to overpower 
any shed upon the spot where we stood by your distant illumination : whereas 
on the 25th, when the night was much darker, the light cast from the tempo- 
rary light-house at Purfleet, in which your apparatus was fixed, was so great 
that a distinct shadow was thrown upon the wall by any object interposed. 
Not the slightest trace of any such shadow, however, could be perceived when 
your light was extinguished, and any of the other lights were exposed in its 
place. 
“ In like manner on the evening of the 25th it was remarked by all the party 
at the Trinity Wharf, that, in whatever direction your light was turned, an im- 
mense coma, or tail of rays, similar to that produced by a beam of sun-light in a 
dusty room, but extending several miles in length, was seen to stream off from 
the spot where we knew the light to be placed, although, owing to the reflector 
being turned too much on one side, the light itself was not visible. 
“Now, last night there was none of this singular appearance visible; but 
whether this was caused by the presence of the moonlight, or by the absence 
of the haze and drizzling rain which fell during the evening of the 25th, I can- 
not say. I had hoped that the appearance alluded to was to prove a constant 
accompaniment to your light, in which case it might, perhaps, have been 
turned to account for the purposes of light-houses. If in hazy or foggy 
weather this curious effect of reflected light from the atmosphere be constant, 
it may help to point out the position of light-houses, even when the distance 
of the observer is so great that the curvature of the earth shall render it impos- 
sible for him to see the light itself. 
“ The following experiments tried last night were the same as those of the 
25th, and certainly no comparative trials could be more fairly arranged. 
