NATURAL PHILOSOPHY. 125 



the most complicated designs of very nearly three-fourths of the ex- 

 pense, and in others certainly more than half. We shall be able, 

 moreover, to correct and vary our designs by a few strokes of the 

 brush ; and their slight cost will permit a more frequent renewal. 



IMPROVEMENT IN LIGHTNING RODS. 



Two patents have been recently granted, for improved lightning- 

 rods, both having for their object such a construction of the point as 

 shall cause it to be left tolerably well pointed even after it has been 

 partially melted by an ele'ctric discharge. The principle in both is 

 the use of metals or metallic alloys of different degrees of fusibility ; 

 the most fusible forming the upper end or outer surface of the point. 

 In one of these points the device consists in completely covering or 

 coating the inner or main point with a pointed metallic sheath of a 

 more fusible metal or alloy ; and this again with another still more 

 fusible ; and this, if desired, with still another. In case of a stroke of 

 lightning sufficiently powerful to melt the point, as not unfrequently 

 happens, from its small mass and the great heat produced where 

 the fluid passes from the air to the conductor, the external sheath 

 is to act as a protector to the point of more infusible alloy within, 

 by absorbing the heat as it melts off, leaving the latter sufficiently 

 perfect still to act as a point. 



The other plan is a modification in which the unequally fusible 

 metals or alloys, instead of being formed as a succession of sheaths, 

 are made solid, and connected one above another by oblique joints or 

 faces, the inclination being towards the square or angular corner of 

 the rod, so that when one section is removed the one next below will 

 be left with a sharp point on that corner. Patent Office Report, 1852. 



ON THE ACTION OF ELECTRICITY UPON THE MAGNETIC NEEDLE. 



There are several methods of measuring the intensity of electrical 

 currents : the most common is the magnetised needle, which has the 

 property of placing itself transversely to the direction followed by 

 the positive electricity following the conductor placed near the nee- 

 dle. It was by making the discovery of this mysterious action, that 

 (Erstedt, the natural philosopher, of Copenhagen, gained the brightest 

 scientific glory achieved during this century; and it was his great 

 good fortune to see before his death, this leading principle serve 

 as the foundation on which the science of electro-magnetism is 

 reared. The action of electricity in motion upon the magnetised 

 needle is such, that if the observer would identify himself with 

 the current (and suppose it entered his body at his feet and 

 left it by his head,) he would find the needle form a cross with 

 him, and point its south pole to the left. As the needle, however, 

 is continually struggling with the action of the earth, it does not com- 

 pletely yield to the current, which, were the earth impotent, would 

 place it at right angles with the current, but generally it takes an ob- 



