NATURAL PHILOSOPHY. 135 



round shot can reach, he may either point the gun by his eye, may employ a 

 common opera-glass, or he may use a small telescope, which, if required, 

 might be fixed to a post. 



In laying guns by means of a telescope some little difficulty may occur 

 from the foci of the sights and the object not being the same. The difficulty 

 can be much diminished by placing the looking-glass at a greater distance 

 behind the gun. In fact, with a simple inverting telescope of very low power, 

 or with a common opera-glass, a very moderate distance will render both 

 objects sufficiently distinct. 



The angular position and elevation of the gun must be adjusted by direc- 

 tions from the officer to the men attending the gun. These adjustments must 

 be contrived by screws, or other means, so as to be made by the men when 

 screened from direct fire. 



When the officer is satisfied that all the guns are well laid, he must then 

 turn to a telescope, attached vertically to the parapet. Fixed to the telescope 

 by an arm reaching above the parapet must be another small looking-glass, 

 having an angular motion on its horizontal axis. This telescope may consist 

 of a single lens of from three to eight feet focus, aud have attached to its eye- 

 glass a small prism to turn the vertical rays into a horizontal direction. 



The officer, having adjusted his telescope on the point he is battering, may 

 then observe the united effect of all the guns ; or he may cause them to be 

 fired in succession, waiting between each shot until the smoke has cleared 

 away, in order that he may judge of the precision with which each gun has 

 been laid. 



The plan of seeing round a corner by means of a small bit of looking-glass 

 has been long known and described in books on the amusements of science. 

 A repetition of the combination constitutes the toy by which children are 

 surprised to find they can see through a deal board. In a different form, by 

 means of an inclined mirror concealed within the tube, the frequenter of the 

 theatre points his glass in one direction whilst he surveys the real object of 

 his attraction in another. Such a telescope, when used behind a wall or a tree, 

 becomes a safe reconnoitring telescope. 



ON SO^tlE OF THE PEIXCTPAL CAUSES OF ATMOSPHEEIC ELEC- 



TEICITY. BY M. BECQUEEEL. 



The causes which constantly furnish the air with an excess of positive and 

 the earth with an excess of negative electricity excesses which are capable 

 of giving rise to storms and other phenomena under certain conditions are 

 still unknown. In studying this question some years ago, I observed the 

 electrical effects produced hi the tissues of plants, and at the contact of these 

 plants- with the soil ; in this contact the soil is constantly positive, and 

 the plant negative, whatever may be the part of the plant put hi metallic 

 communication with it. I then indicated this evolution of electricity as one 

 of the causes of the electricity of the atmosphere. In repeating these experi- 

 ments, a year ago, I was struck by the anomalies manifested, in operating on 

 the margin of a river, or in the river itself, or at a certain distance, near the 



