now measuring on the Continent 197 



MM. Brousseaud and Nicollet employed, for distances from 

 sixty to eighty geographic miles, half a French pound, and 

 even between a quarter and a half, finding that quantity ample 

 for visibility. By experiments made at St. Preuil, by M. 

 Nicollet, we learn that, at distances of twenty geographic 

 miles, the inflammation of so small a quantity of powder as the 

 sixteenth of an ounce was distinctly seen by the naked eye ; and 

 that even the flash of the priming of a musket was visible, at the 

 same distance, by the aid of glasses. It is related by the Baron 

 de Zach, (Cor. Ast. 1822, p. 273,) that, in experiments made 

 in 1804, the illumination made by six ounces of powder fired 

 on Mount Brocken, was seen without glasses on the Keulen- 

 berg, a distance of one hundred and thirty geographic miles, 

 although^ the Mount Brocken itself is not visible from Keu- 

 lenberg, being below its horizon ; a fact which, related upon 

 less authority, might appear almost incredible. More general 

 experience, however, has shown that the illumination alone, dif- 

 fused by the inflammation, cannot be relied upon as a signal, 

 but that it is necessary that the inflammation itself should be 

 actually within the view. 



The knowledge of the least quantity of powder that will secure 

 visibility becomes the more important, when, from the absence 

 of hills of sufficient height, it is necessary to give the signals an 

 artificial elevation by means of rockets : this was the case on 

 the whole extent of the arc between Brest and Strasbourg, 

 except at a single station, Donon, where the arc is crossed by 

 the chain of the Vosges ; at all the other stations of this parallel 

 in France rockets were employed, varying in diameter from 15 

 to 18 and 24 French lines, and calculated to carry eight ounces 

 of powder to the respective heights of 200, 300, and 400 toises, 

 previous to its explosion, which was rendered the more instan- 

 taneous by a contrivance in the rocket to carry the spark, by 

 which it is fired, at once into the middle of the powder. 



In a country, in which there are no hills of sufficient height to 

 direct the choice of stations, and where, in order to avoid the too 

 great multiplication of posts, the stations are necessarily taken 

 not visible from each other, the selection may still, in some 

 degree, be guided by the consideration, that those places are 

 most eligible as stations of observation, which have a low hori- 



