In another article, Dunbar gives a "Description of a singular phenomenon seen at Baton Rouge" in the spring of 1800.
His fourth contribution consists in extracts from a letter dated Aug. 22, 1801, which he wrote to Jefferson, relative "to fossil bones found in Louisiana, and to Lunar Rainbows observed West of the Mississippi." In this letter, he referred to an account of "Dr. Hooks' scheme of a telegraphy, in the year 1684," which he intended to transmit to Jefferson, but found himself anticipated in that communication by a paper in the first volume of the London Philosophical Magazine. He also directs Jefferson's attention to "a certain phenomenon at sunset,"—the yellow orange color of the Eastern clouds, which ascends as the sun descends—upon which he makes certain observations and explanations, and suggestions for further investigations by philosophers.
With this letter there was enclosed a fifth contribution to the Transactions. This article is entitled, "Meteorological Observations made by William Dunbar, Esq., at the Forest, four miles east of the Mississippi, in Latitude 31° 28´ North, and in Longitude 91° 30´ west of Greenwich, for the year 1800; with remarks on the state of the winds, weather, vegetation, etc., calculated to give some idea of the climate of the country." In this article he says, "the frequent and rapid changes in the state of the weather in this climate furnish an excellent opportunity of verifying the vulgar opinion of the moon's pretended influence at her conjunctions, oppositions and quadratures; but truth compels me to say (what probably may be said of many similar persuasions) that after a continued and scrupulous attention to this object, I have not discovered any such regularity of coincidences, which might justify the reverence with which those traditional maxims are at this day received." After discussing a method of manufacturing ice by artificial means, he concludes this communication with the following observations on the storms of the Gulf Coast region: "It is evident that the circular course of the vortex followed that of the sun's apparent diurnal motion.—It is possible that if similar observations are made upon all hurricanes, tornadoes and whirlwinds they will be found universally to consist of a vortex with a central spot in a state of profound calm."
Dunbar's next letter that is preserved in the Jefferson Papers is one to John Vaughan bearing the date of March 21, 1802. In this the writer says that he envies Vaughan's "happiness at the discovery of a complete skeleton of a mammoth." He makes some observations on the species to which this mammoth belongs and refers to recent discoveries of a similar nature in the interior of Asia and Borneo. He gives the results of recent geological observations on the nature of the soil and the stratification of the same as shown by the banks of the Mississippi at Natchez; also a discussion of stones, rocks, ores, mineral waters, petrifaction, etc. He requests Vaughan to inform Dr. Bartram that since writing him last, he has made several new discoveries of a botanical and zoological nature which he here describes.
This letter also shows that Dunbar was one of the first Mississippians to resort to inoculation for protection against small-pox. He asked Vaughan to send him some fresh vaccine virus and stated that six children in his own family had never "had that disease, besides a lengthy list of Black people, both young and old." Vaughan complied with this request by sending the virus and asked Jefferson to do likewise, stating that "the Vaccine inoculation gathers strength hourly, no respectable practitioner (of Philadelphia) opposes it."
January 15, 1803 Dunbar wrote to Jefferson: "Bad health which has endured above twelve months has withheld much of my attention from Philosophic objects, a favorable change having lately taken place, I perceive with satisfaction that my mind and body are both recovering their former tone and now again enjoy the pleasing prospect of dedicating my leisure hours to my favorite amusements."
Dunbar's next contribution to the Transactions was entitled, "Abstract of a communication from Mr. Martin Durale, relative to fossil bones, etc., of the County of Opelousas, west of the Mississippi to Mr. William Dunbar of the Natchez," etc. In this account Dunbar, in referring to certain phenomena makes use of the following expression, which has characterized the true philosophers of all ages, "I have never observed them without endeavoring to ascertain the cause of them."
This communication was accompanied by "pretty full vocabularies of the tongues of two Indian nations of that country," to which "was added a sketch of the religion or superstition of these people." In this connection, Dunbar says, "From several other quarters I have used some efforts to draw similar information, but am hitherto disappointed." He also makes mention of a letter which he had just received from Sir Joseph Banks with an extract from the Transactions of the Royal Society.
January 28, 1804, Dunbar wrote to Jefferson transmitting his seventh and eighth contributions to Volume Six of the Transactions, while an extract from his letter was published as a ninth contribution. His seventh article was entitled a "Description of the river Mississippi and its Delta, with that of the adjacent parts of Louisiana." In this he gives a table of the mean altitude of the waters of the Mississippi at Natchez, from the lowest ebb to the highest elevation for the first and fifteenth of each month in the year. It also contains a good account of overflows and some philosophical reflections on the velocity, banks, currents, deposits and depth of the river and the effects of confining it to its channel. In speaking of the overflow lands he says, "although no successful attempt is likely to be made in our day, yet posterity will reclaim" them. He discusses the methods used in Holland and in Egypt, and makes several speculations as to the method that will probably be successful. This sketch, he says, in conclusion, "is the result of occasional observation for a series of years and of scattered information collected from various sources, probably often uncertain, from a cause which is unfortunately, too general; viz: the extreme inattention of persons, even of some education to the most curious phenomena passing daily under their review."