1 64 LIGHT SCIENCE FOR LEISURE HOURS. 



produces, in the first place, by the mere difference of 

 temperature, important atmospheric disturbances. The 

 difference is so great, that Franklin suggested the use 

 of the thermometer in the North Atlantic Ocean as a 

 ready means of determining the longitude, since the 

 position of the Gulf Stream at any given season is 

 almost constant. 



But the warmth of the stream itself is not the only 

 cause of atmospheric disturbance. Over the warm 

 water vapour is continually rising ; and, as it rises, is 

 continually condensed (like the steam from a loco- 

 motive) by the colder air round. ' An observer on 

 the moon,' says Captain Maury, ' would, on a winter's 

 day, be able to trace out by the mist in the air the 

 path of the Gulf Stream through the sea.' But what 

 must happen when vapour is condensed? We know 

 that to turn water into vapour is a process requiring 

 that is, using up a large amount of heat ; and, con- 

 versely, the return of vapour to the state of water sets 

 free an equivalent quantity of heat. The amount of 

 heat thus set free over the Gulf Stream is thousands 

 of times greater than that which would be generated 

 by the whole coal supply annually raised in Great 

 Britain. Here, then, we have an efficient cause for 

 the wildest hurricanes. For, along the whole of the 

 Gulf Stream, from Bernini to the Grand Banks, there 

 is a channel of heated that is, rarefied air. Into 

 this channel, the denser atmosphere on both sides is 

 continually pouring, with greater or less strength. 

 When a storm begins in the Atlantic, it always makes. 



