1411. Season of Thunder. Although in this country thunder may happen at any time of the year, yet the months of July and August are those in which it may almost certainly be expected. Its visitations are of very uncertain continuance; sometimes only a few peals will be heard at any particular place during the whole season; at other times the storm will return, at intervals of three or four days, for a month, six weeks, or even longer; not that we have violent thunder in this country directly vertical in any one place so frequently in any year, but in many seasons it will be perceptible that thunder-clouds are formed in the neighbourhood, even at these short intervals. Hence it appears, that, during this particular period, there must be some natural cause operating for the production of this phenomenon, which does not take place at other times. This cannot be the mere heat of the weather, for we have often a long succession of hot weather without any thunder; and, besides, though not common, thunder is sometimes heard in the winter. As therefore the heat of the weather is common to the whole summer, whether there is thunder or not, we must look for the causes of it in those phenomena, whatever they are, which are peculiar to the months of July, August, and the beginning of September. Now, it is generally observed, that from the month of April, an cast or south-east wind generally takes place, and continues with little interruption till towards the end of June. At that time, sometimes sooner, and sometimes later, a westerly wind takes place; but as the causes producing the east wind are not removed, the latter opposes the west wind with its whole force. At the place of meeting, there are naturally a most vehement pressure of the atmosphere, and friction of its parts against one another; a calm ensues, and the vapours brought by both winds begin to collect and form dark clouds, which can have little motion either way, because they are pressed almost equally on all sides. For the most part, however, the west wind prevails, and what little motion the clouds have is towards the east: whence the common remark in this country, that 'thunder-clouds move against the wind.' But this is by no means universally true: for if the west wind happens to be excited by any temporary cause before the natural period when it should take place, the east wind will very frequently get the better of it; and the clouds, even although thunder is produced, will move westward. Yet in either case, the motion is so slow, that the most superficial observers cannot help taking notice of a considerable resistance in the atmosphere.
