26 NOTE ON A LIGHTNING DISCHARGE. 



On looking at the sketch it willj^e noticed that five of the 

 trenches start from excavations at some distance from the bases 

 of the trees. There were no signs of tunnels connecting them 

 with the bases of the trees, so that it would appear that the 

 lightning must have branched before it reached the ground. 



Another point shown by the sketch is that the trees must 

 have been nearly, if not quite, in the centre of the charged area 

 of ground. This is indicated by the even way in which the 

 furrows radiate from the bases of the trees. The area of ground 

 covered by the furrows — that is to say, which is enclosed by 

 drawing a line so as to connect their outer extremities, is about 

 3,500 square feet in extent. 



There had been heavy rain before this particular flash 

 (wliich occurred at a few minutes to six o'clock in the evening) 

 and the surface of the groimd must have been very wet. Over 

 one inch of rain fell during the thunder-storm, and certainly 

 more than half of it fell prior to the trees being struck'. The 

 rainfall for the week preceding was as follows : 



February 25th ... ... ... 2-15 inches. 



26th 0-47 „ 



27th 0-25 „ 



28th Nil „ 



March 1st 



„ 2nd 



„ 3rd 



„ 4th 



„ 5th 102 „ 



It will be seen that there were three wet days, with a total 

 rainfall of 2'87 inches, followed by five dry days. Before the 

 rain commenced on the afternoon of Sunday, the 5th March, the 

 surface of the ground for some few inches in depth must there- 

 fore have been fairly dry, and at the time of the flash it is 

 probable that there was still a layer of dryish earth between the 

 wet surface and the saturated subsoil. It was through this top 

 layer of wet earth that the lightning jiassed, and the less moist 

 layer beneath it seems to have proved an effective insulator. 



A study of this discharge tends to cast doubt on many of 

 the ideas respecting lightning and lightning-conductors. It 

 seems to show that the charge is strictly confined to the surface 

 of the ground, and that the discharge under certain circumstances 

 si)reads out disruptively on the charged surface and does not 

 penetrate into the earth, although, as in this case, it wo\ild only 

 have had to go through a few inches of damp earth before 

 reaching the water-level. Instead of this it will be seen that 

 some of the furrows ran for a distance of over fifty feet, just 



