To Thomas Percival
Extract printed in Memoirs of the Literary and Philosophical Society of Manchester, II (1785), 110–13.
The following letter marks the beginning, as far as we know, of Franklin’s interest in the question of where and how raindrops grow.8 This question, one of the key ones in modern cloud physics, had been posed two years earlier in a form that threw investigators off the track for years to come: Dr. William Heberden had announced to the Royal Society that rainfall at the top of a tall building was markedly less than at the bottom.9 Thomas Percival sent Franklin, with his letter above of May 16, a paper that propounded a theory to account for this phenomenon; and Franklin is here replying. A charged particle in a cloud, Percival argued, is attracted to neighboring particles of opposite charge, with which it coalesces to form drops that grow until they are large enough to fall. Their direction of fall, toward the center of the earth, makes them converge and, drawing in other drops by collision and electrical attraction, become steadily larger as they approach the ground.1
Franklin accepted as fact that the amount of rainfall varies with altitude because falling drops increase in size. But he tentatively advanced an explanation that he preferred to Percival’s: the increase is due to the drops’ acquiring water vapor that condenses on them from the air through which they fall. He pointed out the objection to this theory at the end of the extract, and advised waiting for more evidence. He could not guess that the evidence would eventually demolish the original premise that drops grow after leaving the cloud.2 Neither could he guess that he and Percival had each identified one of the two factors, condensation and coalescence, that account for the formation of drops in the cloud.
[June ?, 17713]
On my return to London I found your favour, of the sixteenth of May (1771). I wish I could, as you desire, give you a better explanation of the phaenomenon in question, since you seem not quite satisfied with your own; but I think we want more and a greater variety of experiments in different circumstances, to enable us to form a thoroughly satisfactory hypothesis. Not that I make the least doubt of the facts already related, as I know both Lord Charles Cavendish, and Dr. Heberden to be very accurate experimenters: but I wish to know the event of the trials proposed in your six queries; and also, whether in the same place where the lower vessel receives nearly twice the quantity of water that is received by the upper, a third vessel placed at half the height will receive a quantity proportionable.4 I will however endeavour to explain to you what occurred to me, when I first heard of the fact.
I suppose, it will be generally allowed, on a little consideration of the subject, that scarce any drop of water was, when it began to fall from the clouds, of a magnitude equal to that it has acquired, when it arrives at the earth; the same of the several pieces of hail; because they are often so large and weighty, that we cannot conceive a possibility of their being suspended in the air, and remaining at rest there, for any time, how small soever; nor do we conceive any means of forming them so large, before they set out to fall. It seems then, that each beginning drop, and particle of hail, receives continual addition in its progress downwards. This may be several ways: by the union of numbers in their course, so that what was at first only a descending mist, becomes a shower; or by each particle in its descent through air that contains a great quantity of dissolved water, striking against, attaching to itself, and carrying down with it, such particles of that dissolved water, as happen to be in its way; or attracting to itself such as do not lie directly in its course, by its different state with regard either to common or electric fire;5 or by all these causes united.
In the first case, by the uniting of numbers, larger drops might be made, but the quantity falling in the same space would be the same at all heights; unless, as you mention, the whole should be contracted in falling, the lines described by all the drops converging, so that what set out to fall from a cloud of many thousand acres, should reach the earth in perhaps a third of that extent, of which I somewhat doubt. In the other cases we have two experiments.
1. A dry glass bottle, filled with very cold water, in a warm day, will presently collect from the seemingly dry air that surrounds it, a quantity of water that shall cover its surface and run down its sides, which perhaps is done by the power wherewith the cold water attracts the fluid, common fire that had been united with the dissolved water in the air, and drawing that fire through the glass into itself, leaves the water on the outside.
2. An electrified body left in a room for some time, will be more covered with dust than other bodies in the same room not electrified, which dust seems to be attracted from the circumambient air.
Now we know that the rain, even in our hottest days, comes from a very cold region. Its falling sometimes in the form of ice, shews this clearly; and perhaps even the rain is snow or ice when it first moves downwards, though thawed in falling: And we know that the drops of rain are often electrified. But those causes of addition to each drop of water, or piece of hail, one would think could not long continue to produce the same effect; since the air, through which the drops fall, must soon be stript of its previously dissolved water, so as to be no longer capable of augmenting them. Indeed very heavy showers, of either, are never of long continuance; but moderate rains often continue so long as to puzzle this hypothesis: So that upon the whole I think, as I intimated before, that we are yet hardly ripe for making one.
8. As early as 1749 he had speculated about their origin, and as late as 1784 he gave further thought to the precipitation of snow, hail, and rain: above, III, 369–71; Smyth, Writings, IX, 215–16.
9. Phil. Trans., LIX (1769), 359–62. For Heberden see above, VIII, 281 n.
1. Percival subsequently published the paper in his Philosophical, Medical and Experimental Essays... (London, 1776), pp. 109–28.
2. That discovery, oddly enough, was made by BF’s great-grandson, Alexander Dallas Bache, who demonstrated that vertical and horizontal wind currents, not height, account for the varying quantities of rain at different altitudes. Bache’s work opened the way to the modern conclusion that drops form in the cloud and rarely grow larger as they fall. See William E. Knowles Middleton, A History of the Theories of Rain... (New York, ), pp. 98–9, 168–70.
3. BF returned from the north on or before June 1 (see the résumé of Williams’ journal above, under May 28), and we are assuming that he answered Percival’s letter during the month.
4. For Cavendish see above, X, 41 n. Percival’s paper ended with six queries for further experimentation, and elsewhere in it he mentioned that a measuring vessel, when placed some fifty feet above another, received just twice as much rain: Percival, op. cit., pp. 125–8, 112 n.
5. BF had long distinguished between electrical and common “fire,” both of which he considered to be fluids; see above, III, 366, 368–70; V, 146–7; VII, 185–9; X, 38–41, 49–50. His common fire, which he conceived of as an imponderable fluid rather than a property of matter, was a forerunner of Lavoisier’s later “caloric” fluid. See Abraham Wolf, A History of Science, Technology and Philosophy in the Eighteenth Century (2nd ed. revised; 2 vols., New York, ), I, 177–8.