To Alexander Small5
MS not found; reprinted from Benjamin Franklin, Experiments and Observations on Electricity (London, 1769), pp. 381–3.6
May 12, 1760.
Agreeable to your request, I send you my reasons for thinking that our North-East storms in North-America begin first, in point of time, in the South-West parts:7 That is to say, the air in Georgia, the farthest of our colonies to the South-West, begins to move South-Westerly before the air of Carolina, which is the next colony North-Eastward; the air of Carolina has the same motion before the air of Virginia, which lies still more North-Eastward; and so on North-Easterly through Pensylvania, New-York, New-England, &c. quite to Newfoundland.
These North-East storms are generally very violent, continue sometimes two or three days, and often do considerable damage in the harbours along the coast. They are attended with thick clouds and rain.
What first gave me this idea, was the following circumstance. About twenty years ago, a few more or less, I cannot from my memory be certain, we were to have an eclipse of the moon at Philadelphia, on a Friday evening, about nine o’clock. I intended to observe it, but was prevented by a North-East storm, which came on about seven, with thick clouds as usual, that quite obscured the whole hemisphere. Yet when the post brought us the Boston news-paper, giving an account of the effects of the same storm in those parts, I found the beginning of the eclipse had been well observed there, though Boston lies N. E. of Philadelphia about 400 miles. This puzzled me, because the storm began with us so soon as to prevent any observation, and being a N. E. storm, I imagined it must have began rather sooner in places farther to the North Eastward, than it did at Philadelphia. I therefore mentioned it in a letter to my brother who lived at Boston; and he informed me the storm did not begin with them till near eleven o’clock, so that they had a good observation of the eclipse: And upon comparing all the other accounts I received from the several colonies, of the time of beginning of the same storm, and since that of other storms of the same kind, I found the beginning to be always later the farther North-Eastward. I have not my notes with me here in England, and cannot, from memory, say the proportion of time to distance, but I think it is about an hour to every hundred miles.
From thence I formed an idea of the cause of these storms, which I would explain by a familiar instance or two. Suppose a long canal of water stopp’d at the end by a gate. The water is quite at rest till the gate is open, then it begins to move out through the gate; the water next the gate is first in motion, and moves towards the gate; the water next to that first water moves next, and so on successively, till the water at the head of the canal is in motion, which is last of all. In this case all the water moves indeed towards the gate, but the successive times of beginning motion are the contrary way, viz. from the gate backwards to the head of the canal. Again, suppose the air in a chamber at rest, no current through the room till you make a fire in the chimney. Immediately the air in the chimney being rarefied by the fire, rises; the air next the chimney flows in to supply its place, moving towards the chimney; and, in consequence, the rest of the air successively, quite back to the door. Thus to produce our North-East storms, I suppose some great heat and rarefaction of the air in or about the Gulph of Mexico; the air thence rising has its place supplied by the next more northern, cooler, and therefore denser and heavier, air; that, being in motion, is followed by the next more northern air, &c. &c. in a successive current, to which current our coast and inland ridge of mountains give the direction of North-East, as they lie N. E. and S.W.
This I offer only as an hypothesis to account for this particular fact; and, perhaps, on farther examination, a better and truer may be found. I do not suppose all storms generated in the same manner. Our North-West thundergusts in America I know are not; but of them I have written my opinion fully in a paper which you have seen.8 I am, &c.
5. Alexander Small (1710–1794), a Scots army surgeon, whose home was in London, was a member of the Society of Arts, a correspondent of learned societies in both Europe and America, and a contributor of essays “on agricultural and physiological improvements.” He invented the chain plow. He and BF were both members of the Committee on Agriculture which the Society set up in 1760. Gent. Mag., LXIV (1794), 864–5; Derek Hudson and Kenneth Luckhurst, The Royal Society of Arts 1754–1954 (London, 1954), p. 63. BF and Small corresponded on a wide range of topics: grass seed, gout, hospital ventilation, and free trade. BF mentioned him in 1764 as one of the few friends to whom he had given a copy of his “Parable against Persecution.” See above, VI, 117.
6. Printed as Letter XXXVI in the 1769 and 1774 editions of Exper. and Obser.
7. For earlier discussions of this topic, in the last of which the same facts and analogies were used, see above, III, 149, 392–3 n, 463–5.