How Benjamin Franklin Transformed a Simple Observation into a Discovery

Forrest M. Mims III

Benjamin Franklin is the best known and most ingenious of all American amateur scientists. His genius was embellished by an uncommon ability to transform seemingly simple observations into discoveries.

How Franklin transformed his observations of the Gulf Stream into important scientific findings was the subject of a previous Editorial in The Citizen Scientist ("Benjamin Franklin's Study of the Gulf Stream," 23 September 2005).

A much simpler observation by Franklin became as important as his Gulf Stream work, for it was among the first recorded instance of a significant reduction of sunlight by a major volcano eruption. Franklin had no measurement devices to record how sunlight was reduced by the cloud from the eruption. Instead, he relied on a simple observation with a glass lens. While Franklin's observation can be summarized in a few sentences, that does not do it justice. The discovery comes alive when one reads about it in Franklin's own words:

During several of the summer months of the year 1783, when the effect of the sun's rays to heat the earth in these northern regions should have been greater, there existed a constant 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 sun seemed to have little effect towards dissipating it, as they easily do a moist fog, arising from water. They were indeed rendered so faint in passing through it, that when collected in the focus of a burning glass they would scarce kindle brown paper. Of course, their summer effect in heating the earth was exceedingly diminished.

Hence the surface was early frozen; Hence the first snows remained on it unmelted, and received continual additions. Hence the air was more chilled, and the winds more severely cold.

Hence perhaps the winter of 1783-4, was more severe, than any that had happened for many years.

The cause of this universal fog is not yet ascertained. Whether it was adventitious to this earth, and merely a smoke, proceeding from the consumption by fire of some of those great burning balls or globes which we happen to meet with in our rapid course round the sun, and which are sometimes seen to kindle and be destroyed in passing our atmosphere, and whose smoke might be attracted and retained by our earth; or whether it was the vast quantity of smoke, long continuing; to issue during the summer from Hecla in Iceland, and that other volcano which arose out of the sea near that island, which smoke might be spread by various winds, over the northern part of the world, is yet uncertain . It seems however worth the enquiry, whether other hard winters, recorded in history, were preceded by similar permanent and widely extended summer fogs. Because, if found to be so, men might from such fogs conjecture the probability of succeeding hard winter, and of the damage to be expected by the breaking up of frozen rivers in the spring; and take such measures as are possible and practicable, to secure themselves and effects from the mischiefs that attended the last.

Passy, May 1784.

This text is from Franklin's paper scanned by the Dartmouth College Libraries and published online here. I have corrected several minor scanning errors and changed the old English letter f to s. The paper was published in (from the Dartmouth scan) MEMOIRS OF THE LITERARY AND PHILOSOPHICAL SOCIETY OF MANCHESTER, SECOND EDITION LONDON: PRINTED FOR T. CADWELL IN THE STRAND MD CCLXXXIX.The full title of the paper, again from the Dartmouth scan, is 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.

Volcanologists have noted that Franklin's attribution of the fog to Hecla was incorrect, for that volcano erupted in 1768. The most likely cause of the dry fog reported by Franklin and others was Lakigiger, which in 1783 emitted possibly the largest amount of lava ever reported by human observers.

Careful research has turned up various other contemporary reports that back up Franklin's 1783 observation. Perhaps the most comprehensive is, "Volcanic eruptions dry fogs and the European palaeoenvironmental record: localised phenomena or hemispheric impacts?" by J.P. Grattan and F.B. Pyatt. This paper also discusses other eruptions.

While all the reports about the atmospheric effects of the 1783 eruption are interesting, none are so elegant as the application by Benjamin Franklin of a simple burning glass to quantify as best he could the rare phenomenon that he observed and reported. May we all be so resourceful should we happen to be present during an important natural event.