ON AQUEOUS AND IGNEOUS METEORS. 715 



escaped with the loss of a telescope, which he held in his hand, and M'ith 

 the sensation of a violent pressure on his head and on his back. A large stone 

 was forced out of the wall near them, and thrown into the room, and some 

 other similar eflects were observed, which marked the progress of the explosion. 



For guarding against accidents so dreadful, Dr. Franklin's great invention 

 of metallic conductors may be very advantageously employed: for, when pro- 

 perly fixed, they afford a degree of security which leaves very little room for 

 apprehension. A conductor ought to be continued deep into the earth, or con- 

 nected with some well or drain; it should be of ample dimensions, and where 

 smallest, of copper, since copper conducts electricity more readily than iron. 

 In one instance a conductor of iron, four inches wide,and half an inch thick, 

 appears to have been made red hot by a stroke of lightning. It seems to be 

 of some advantage that a conductor should be pointed, but the circumstance 

 is of less consequence than has often been supposed. Mr. Wilson exhibited 

 some experiments in which a point was struck at a greater distance than a 

 ball, and therefore argued against the employment of pointed conductors. 

 Mr. Nairne, on the contrary, showed that a ball is often struck in preference 

 to a point. But it has been observed, that if a poiat attracts the lightning from 

 a greater distance, it must protect a grea/ter extent of building. It is easy 

 to show, by hanging cotton or wool on a conductor, that a point repels light 

 electrical bodies, and that a pointed conductor may, therefore, drive away 

 some fleecy clouds; but this effect is principally derived from a current of 

 air repelled by the point; and such a current could scarcely be supposed to 

 have any perceptible efiect on clouds so distant as those which are concerned 

 in thunder storms. In order to escape personal danger in a thunder storm, 

 the best precautions are, to avoid eminences, and all exposed situations, as 

 well as a near approach to conductors. The neighbourhood of windows, look, 

 ing glasses, fire places, and trees, must always be considered as hazardous. 



It has been supposed that a sudden condensation of the air, arising from 

 cold, accompanied by a deposition of moisture, and propagated by a con- 

 tinuation of the cause, by means of the cold occasioned by expansion, pro- 

 duces frequently the noise of thunder, without any lightning, and without 

 any electrical agitation; but it does not appear that the opinion is well 

 established. 



