24 



given off, which being interpreted must mean either vapour or carbon dioxide, for 

 chemists state that there is no chemical action and chemically no gases are given 

 off; while bacteriologists say that some carbon dioxide may be given off in case 

 the hay or grain heat somewhat, which they do unless comparatively dry, but no 

 other gas could be. Now carbon dioxide being heavier than air at the same 

 temperature could not rise more than a few inches above the mow — Just 

 enough to allow it to become cool. Hence the explanation depends upon the water 

 vapour. What has always puzzled us is to know how the vapour could escape from 

 the barn fast enough to form a column, even in calm weather, much less if some 

 wind is blowing. Besides, when a thunderstorm is approaching the atmosphere- 

 is usually so humid as to be oppressive, and air escaping from the barn would 

 probably be drier than that outside. This would be particularly true when the- 

 rain begins to fall, for then the outside air is saturated, and hence any that 

 might escape from the barn would tend to make that above drier than else- 

 where. In view of these considerations we cannot see any possibility of the vapour 

 in the barn or from the barn having any effect whatever in causing lightning- 

 strokes. But we have always hesitated to attack the theory because we had noth- 

 ing to offer in its stead, except that hay and grain with their light leaves and chaff 

 are easily ignited, and as they touch the barn in thousands of places, even a small 

 s])ark anywhere in the building is almost sure to fire it. This, however, would 

 not account for strokes on these barns being more frequent than on empty ones, if 

 such be the case. 



Perhaps the greater inflammability due to the presence of the hay and grain 

 is sufficient explanation. We have seen on page 23, that when a lightning dis- 

 charge occurs there are surgings of electricity on the conductors in that vicinity, 

 which doubtless produce many tiny sparks, any of which occurring in the barn 

 would ignite the dry leaves of the plants. Sometimes in cases of this kind it is 

 said " the barn just seemed to take fire all over " — doubtless a case of many sparks 

 each setting its own little fire. There may sometimes be another cause aggravating 

 the danger, viz., that owing to the heating process in the mow the grain may be 

 nearly hot enough to take fire itself, in which condition it is more easily ignited 

 by a spark. 



How TO EoD A Building. 



Let us now enquire : " AVhen is a building properly rodded ?" The first prob- 

 lem is, what kind of rod shall we use? 



1. Kind of Rod. — Until recently iron and copper were the only two metals 

 thought of in connection with lightning rods. Now aluminum also becomes a com- 

 petitor, as there are rods of this material on the market. For a long time copper was 

 considered the only metal for this purpose, the reason being that it conducts a 

 steady current of electricity more than six times as well as iron, the size of wire 

 being the same in both cases. But this difference may be overcome by using iron 

 wire six times as large in cross-section as the copper. This, however, makes too 

 heavy a cable. But the conductivity of steel is only about seventy per cent as great 

 as that of iron, so that steel rods would have to be considerably heavier than iron 

 to have the same conducting value. As standard copper rods weigh at least three 

 ounces to the foot, the iron cable would have to weigh more than one pound and 

 steel rods almost a pound and three-quarters to the foot to be as good conductors of 

 steady current as the copper rods in general use. Hence it is not practicable to 

 make iron or steel rods that will be as good conductors of steady current as the- 

 copper rods. 



