Background
Jeffries was born on February 5, 1745, in Boston, Massachusetts, the son of David and Sarah (Jaffrey) Jeffries and the great-grandson of an earlier David Jeffries who emigrated from England to Boston in 1677.
Jeffries was born on February 5, 1745, in Boston, Massachusetts, the son of David and Sarah (Jaffrey) Jeffries and the great-grandson of an earlier David Jeffries who emigrated from England to Boston in 1677.
John graduated from Harvard in 1763, studied medicine under home practitioners and in England and Scotland, and received the degree of M. D. from Marischal College, Aberdeen, at the age of twenty-five.
Returning to Boston, Jeffries practiced there until 1771, when he became assistant surgeon on a British naval vessel, serving in that capacity until 1774. When Boston was evacuated he withdrew to Halifax in company with many Loyalists. From 1775 to 1779 he was a surgeon in British military hospitals; he then went to England and was appointed surgeon-major and as such served with British troops in the campaign around Savannah and Charleston. Returning to England he practiced successfully and also became interested in levitation, as it was then called, or aerostation. He seems to have been the first to attempt to gather scientific data of the free air. His observations were made with care and, since he had sufficient means, he was able to secure instruments of high grade. His flights were made with the French aeronaut Blanchard, who received distinguished honors, but it was Jeffries who paid all the bills, accepted the responsibility, and made the observations. Two ascents, the one over London, Tuesday, November 30, 1784, and the other across the English Channel, were made for scientific purposes. The first took place in the presence of the Prince of Wales, the Duchess of Devonshire, and other notables. This was done at the suggestion of Cavendish, who subsequently made a chemical analysis of the air. Twelve observations of temperature, pressure, and humidity were made. These constitute the first scientific data for free air, to a height of 9, 309 feet. The values agree closely with modern determinations. After their descent, they were received with much enthusiasm and were conveyed to Calais, where they were later entertained by the mayor and other dignitaries. Blanchard was given the freedom of the city, and apologies were made to Jeffries because similar honors could not be paid to him without leave from the Court. On January 11, 1785, they reached Paris, and two days afterwards Jeffries was complimented by the King, and on January 15 he dined with Franklin at Passy. The days in Paris were a continuous round of receptions, dinners, and theatre parties. He met Commander John Paul Jones at Franklin's dinner table and each complimented the other for bravery. Returning to Dover, Jeffries was given the freedom of the city and made a Baron of the Cinque Ports, and on March 5, was back in London. Returning to Boston about 1790, Jeffries established a large and profitable practice. Jeffries died on September 16, 1819, in Boston, Massachusetts.
Jeffries was married, first, about 1770, to Sarah Rhoads, by whom he had three children, and, second, September 8, 1787, at London, to Hannah Hunt, by whom he had eleven children. Benjamin Joy Jeffries was a grandson.