48 BOARD OF AGRICULTURE. 



secondary agent in rending bodies asunder and projecting the 

 pieces to a distance is thought to be the elastic force of steam. 

 A temperature of 400° Fahrenheit converts water into steam with 

 a tension of IT atmosphere; a temperature of 500° gives rise to 

 steam with a tension of 45 atmosphere. 



We know that lightning may fuse small metallic wires, or at 

 least render them incandescent ; we know that the heat developed 

 by a stroke of lightning may be competent to fire buildings. 

 Suppose a block of free stone containing moisture in its fissures 

 or between its particles is struck by lightning, steam is at once 

 developed of sufficient tension to shatter the stone into pieces, 

 and project the fragments in all directions. The action of steam 

 is clearly shown in the peculiar and minute division of wood by 

 the passage of lightning. It is split in the direction of its length 

 into "thin laths or still more slender fibres." In the vapor of 

 water suddenly generated at a high temperature we have a force 

 competent to displace the foundations of buildings, to raise and 

 transport heavy masses, and give rise to the other manifestations 

 of power which so frequently accompany a stroke by lightning. 

 The direction of the electric discharge, (whether upwards or 

 downwards or at an oblique angle,) cannot, as matter of course, 

 be determined in those cases in which steam has been the im- 

 mediate agency in producing mechanical effects. 



The magnetic action of lightning cannot be safely disregarded. 

 It always affects the needle of a compass in passing near it, some- 

 times wholly destroys its magnetism, and sometimes reverses its 

 magnetic poles. The manner in which this discovery was made is 

 always read with interest. 



"About the year 1675, two English vessels were sailing in 

 company from London to Barbadoes. Not far from the Bermudas 

 a thunderstroke shattered the mast and rent the sails of one of the 

 ships, while the other sustained no damage. The captain of the 

 latter seeing that his consort had altered her course, as if making 

 for England, asked the cause of this sudden change of purpose, 

 'and found much to his astonishment, that her captain and crew 

 believed themselves to be still following the same course as be- 

 fore. An attentive examination of the compasses of the vessel 

 which had been struck by lightning, showed that the characteristic 

 mark on the compass cards, which before the stroke pointed, as 

 is usual, towards the north, now pointed on the contrary, to the 

 south, showing that the poles had been completely reversed by 



