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      M E M O I R S  

                O F T H E

         L I T E R A R Y

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   PHILOSOPHICAL SOCIETY

.       OF MANCHESTER

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  PRINTED FOR T. CADWELL 
         IN THE  STRAND

         MD C C L X X XIX   
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METEOROLOGICAL IMAGINATIONS and CONJECTURES. By BENJAMIN FRANKLIN, LL.D F. R. S. and acad. reg. Scient. Paris. Soc. etc. Communicated by Dr. PERCIVAL. Read December 22, I784.,

T H E R E seems to be a region higher in the air over all countries, wbere it is always winter, where frost exitfls continua1ly, fince, in the midf of fummer on the furface of the earth, ice falls often from above in the form of hail. Haiftones, of the great weight we fometimes find them, did not probably acquire their mag nitude before they began to dercend. The air, being eight hundred times rarer tnan water, is unable to fupport it but in the fhape of vapour a ftate in which its particles are feparated. As foon as they are condenfed by the cold of the upper region, fo as to form a drop, that drop begins to fa11. If it freezes into a grain of

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ice, that ice defcends. In defcending, both the drop of water, and the grain of ice, are augmented by particles of the vapour they pafs through in falling, and'which they condenfe by their coldnefs, and attach to themfelves. It is pofflble that, in fummer, much of what is rain, when it arrives at the furface of the earth~ might have been fnow, when it began its defcent; but being thawed, in paffng through the warm air near the furface, it is changed from fnow to rain. How imrnenfely cold muft be the original particle of hail, which forms the center of the future hai!ftone, fince it is capable of communicating fufficient cold, if I may fo fpeak, to freeze all the mafs of vapour condenfed round it, and form a lump of perhaps fix or eight ounces in weight !

When, in fummer time, the run is high, and continus long every day above the horizon, his rays ftrike the earth more directly, and with longer continuence, than in the winter; hence, the furface is more heated, and to a greater depth, by the heat of thofe rays.

When rain fa!ls on the heated earth, and falls down into it, it carries down with it a great part of the heat, which by that means defcends ftill deeper.

The mafs of earth, to the depth perhaps of thirty feet, being thus heated to a certain degree,

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continues to retain its heat for fome time. Thus the firft fnows that fall in the bsginning of winter, feldom lie long on the furface, .but are foon melted, and fioon abforbed. After which, the winds that blow over the country on which the finows had fallen, are not rendefed fo cold as they would have been by thofe fnows, if they had remained. And thus the approach of .the feverity of winter is retarded; and the extreme degree of its cold is not always at the time we might expect it, viz. when the fun is at its greateft diftance, and the day fhorteft, but fome time after .that period, according to the Englifh proverb which fays, "as the day lengthens, the cold ftrengthens;" the caufes of refigeration continuing to operate, while the fun returns too flowly and his force continues too weak to counteract them.

During feveral of the fummer months of thc year 1783, when the effect of the fun's rays to heat the earth in these northern regions fhould have been greater, there exifted a conftant fog over all Europe, and great part of North America- This fog was of a permanent nature; it was dry, and the rays of the fun feemed to have little effect towards diffipating it, as they eafily do a moift fog, arifing from water. They were indeed rendcred fo faint in paffing through it, that when collefted in the focus of a burning glafs they woul;l fcarce kindle brown paper.

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Of courfe, their fummer effect in heating the earth was exceedingly diminifhed.

Hence the furface was early frozen;

Hence the firft snows remained on it unmelted, and received continual additions. Hence the air was more chilled, and the winds more feverely cold.

Hence perhaps the winter of 1783-4, was mor fevere, than any that had happened for rnany ycars.

The caufe of this univerfal fog is not yet afcertained. Whether it was adventitious to this earth, and merely a fmoke, proceeding from the confumption by fire of fome of thofe great burning balls or globes which we happen to meet with in our rapid courfe round the fun, and which are fomecimes feen to kindle and be deftroyed in paffng our atmofphere, and whofe fmoke might be attracted and retained by our earth; or whether it was the vaft quantity of fmoke, long continuing; to iffue'during the fummer fiom IIecla in Iceland, and that other volcano which arofe out of the fea near that ifland, which rmoke might be fpread by various winds, over the northern part of the world, is yet uncertain . It feems however worth the enquiry, whether other hard winters, recorded in hiftory, were preceded by fimilar permanent and widely extended fummer fogs. Becaufe, if found to be

                               
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fo, men migbt from fuch.fogs conjecture tbe probability of fucceeding hard winter, and of the damage. to be expected by the breaking up of frozen rivers in the fpring; and take fuch measures as are poffible and practicable, to fecure themfelves and effects from the mirchiefs that attended the laft.

Passy, May 1784.

-------------------------------------------------------------------------------------------------------- Comments by the editors:

Franklin imputed the volcanic fog to Hecla Hecla erupted in 1768 but Lakigiger the long fissure in southeastern Iceland erupted the largest volume of lava in historic time in 1783. This was quite surely the culprit. The text of Franklin's article, which was scanned, was supplied to the editors by the Dartmouth College Libraries

Last modified April 15, 1996 rs