I.

It is probable that Webster’s theory of influenza as related to earthquakes and volcanoes, first published in 1799, was suggested to him by a communication to the Royal Society on the volcanic waves seen at Barbados on the 31st of March, 1761, and on the epidemic of influenza thereafter ensuing all over the island. At Bridgetown, in the afternoon of the 31st of March, 1761, the water in the bay and harbour ebbed and flowed to the extent of eighteen inches or two feet at intervals of eight minutes, and continued to do so for the space of three hours, the oscillation regularly decreasing till night when it was no more observable. These tidal waves were due to volcanic upheavals somewhere; and it was found that the centre of disturbance had been in the Atlantic near the coast of Portugal, and the time some hours earlier than the waves were felt at Bridgetown. The Barbados chronicler proceeds:

“It is very remarkable that since that time the island has been in a very deplorable condition, having suffered under the severest colds that have been ever known. The distress has been so general that I may venture to assert (with confidence) that nineteen twentieths of the inhabitants of the island have felt the effects of the contagion; and to some it has been repeated several times. It has puzzled all the adepts in pharmacy to find out the cause and cure of it. One favourable circumstance has attended it, viz. few have died with it. The Leeward Islands have not escaped, it having raged there more violently and more fatal. His Majesty’s ships have severely felt the effects of it, some of them not being capable of keeping the seas for want of men fit for service. This happening at a season of the year remarkably the healthiest, makes it the more surprising[761].”

This is as good an instance as we shall find, of explaining something sudden, swift, and phenomenal, by something else sudden, swift, and phenomenal, in a purely empirical way and without pausing to ask whether the latter could have been a _vera causa_ of the former. That the influenza came to Barbados in the wake, as it were, of the volcanic waves, had been a common subject of talk among the residents; and that common opinion of the colony had found expression in the paper sent to the Royal Society. The influenza was not only in Barbados, in the Leeward Islands, and in the ships on the West Indian Station, but also in New England and “over the whole country” of the North American Colonies. Dr Tufts, of Weymouth, New England, wrote to Webster that “it began in April, and in May ran into a malignant fever which proved fatal to aged persons. It spread over the whole country and the West India Islands[762].” It was not until some nine months after that influenza appeared in Europe, at first in the east of that continent,--Hungary, Vienna, Breslau, Copenhagen--in February and March, 1762, in central Germany and Scotland in April, in London about the first of May and all over England and Ireland thereafter, but not in France until June and July.

Precisely the same order was followed by the influenza twenty years after: it began in North America in March, 1781, and, says Webster, spread over that continent; it appeared in the East Indies in October and November, 1781, and on the eastern confines of Europe in January, 1782, having been traced from Tobolsk, made a slow progress westwards, and was at its height in London about the end of May or beginning of June. Assuming, says Webster, that the American influenza of 1781 had been continuous with the European of 1782, it must have “passed the Pacific in high northern latitudes,” traversed Siberia and Tartary, and so reached Russia in Europe. In like manner, if the European influenza of 1762 were continuous with the American of 1761, it must have made the circuit of the globe in the same order, as if it were following the first impulse of the volcanic waves across the Atlantic from the coast of Portugal westwards, and so round the earth until it came back to Europe on its eastern frontier. So much may be fairly advanced on the ground of a particular set of facts. But then there were many other facts, both in 1761-62, and in 1781-82. Meanwhile let us take another instance of volcanic waves felt at Barbados six years before, on the same afternoon as the great earthquake of Lisbon.