During the 1760s and early 1770s, Benjamin Franklin spent long periods in Europe as the London agent for the colonies of Pennsylvania, Georgia, Massachusetts and New Jersey.
During one such sojourn he had an opportunity to visit Ireland, and he landed on our shores 229 years ago today, on October 5th 1771.
During his short visit he met virtually everyone of influence on the island, and also spent a day observing the proceedings in the House of Commons, now the Bank of Ireland, across the Green from Trinity.
In one of his many alter egos, that of meteorologist, Franklin was one of the first to investigate the structure of the upper atmosphere. Some 20 years before his Irish visit, by means of his famous experiment with a kite in the middle of a thunderstorm, he demonstrated that lightning was electrical in nature, a momentous discovery at the time.
In fact for many years it comprised virtually all that was known about happenings far above our heads. But now we know much more.
We know, for example, that the atmosphere can be divided into a series of layers, rather like those of an onion, that alternate between warm and cold.
The lowest, adjacent to the Earth, is the troposphere in which we live. It is seven or eight miles deep and contains nearly all the phenomena we know as "weather" - the clouds, thunderstorms and so on. Within it, the temperature decreases with height at an average rate of about two degrees per thousand feet, and in Franklin's time it was assumed that this decrease went on indefinitely.
Not so, however. At the tropo-pause, a boundary dividing the troposphere from the zone above, the stratosphere, the temperature of the air begins to rise, until 30 miles high it is above the freezing point again.
Next in order of height, from about 30 to 60 miles above the Earth, comes the mesosphere, where the temperature again falls with height until the lowest values in the atmosphere are reached, as low as 100 Celsius.
And then in the thermosphere, a final layer which really has no ceiling, temperature increases sharply with height; a hundred miles or so above the Earth the surroundings are warmer than at sea level, and the increase continues farther into space.
The warming at these high levels occurs because of absorption by the atmosphere of certain wavelengths of the sun's radiation.
But then the air is so very tenuous in these regions that very little energy is required to bring about a substantial rise, and indeed the whole concept of temperature as we experience it here on Earth has very little relevance.