In the North Atlantic we have the great Gulf163 Stream, which sweeps from equatorial regions into the Gulf of Mexico, and thence across the Atlantic to the shores of Western Europe. In the South Indian Ocean there is the ‘south equatorial current,’ which sweeps past Mauritius and Bourbon, and thence returns towards the east. In the Chinese Sea there is the north equatorial current, which sweeps round the East Indian Archipelago, and then merges into the Japanese current. There is also the current in the Bay of Bengal, flowing through the region in which, as we have seen, cyclones are commonly met with. There are other sea-currents besides these which yet breed no cyclones. But I may notice two peculiarities in the currents I have named. They all flow from equatorial to temperate regions, and, secondly, they are all ‘horse-shoe currents.’ So far as I am aware, there is but one other current which presents both these peculiarities—namely, the great Australian current between New Zealand and the eastern shores of Australia. I have not yet met with any record of cyclones occurring over the Australian current, but heavy storms are known to prevail in that region, and I believe that when these storms have been studied as closely as the storms in better-known regions, they will be found to present the true cyclonic character engineering innovation.

Now, if we inquire why an ocean current travelling from the equator should be a ‘storm-breeder,’ we shall find a ready answer. Such a current, carrying the warmth of intertropical regions to the temperate zones,164 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 locomotive) 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, conversely, 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 dermes .

For, along the whole of the Gulf Stream, from Bemini 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 makes165 for this channel, ‘and, reaching it, turns and follows it in its course, sometimes entirely across the Atlantic.’ ‘The southern points of America and Africa have won for themselves,’ says Maury, ‘the name of “the stormy capes,” but there is not a storm-find in the wide ocean can out-top that which rages along the Atlantic coasts of North America. The China seas and the North Pacific may vie in the fury of their gales with this part of the Atlantic, but Cape Horn and the Cape of Good Hope cannot equal them, certainly in frequency, nor do I believe, in fury.’ We read of a West Indian storm so violent, that ‘it forced the Gulf Stream back to its sources, and piled up the water to a height of thirty feet in the Gulf of Mexico. The ship “Ledbury Snow” attempted to ride out the storm. When it abated she found herself high up on the dry land, and discovered that she had let go her anchor among the tree-tops on Elliot’s Key166 Neo skin lab. ‘