Little being known to Swedenborg, as indeed little is known to the astronomers of our own time, about Mercury, we find little in the visions relating to that planet which possesses any scientific interest. He asked the inhabitants who were brought to him in visions about the sun of the system, and they replied that it looks larger from Mercury than as seen from other worlds. This of course was no news to Swedenborg. They explained further, that the inhabitants enjoy a moderate temperature, without extremes of heat or cold. 'It was given to me,' proceeds Swedenborg, 'to tell them that it was so provided by the Lord, that they might not be exposed to excessive heat from their greater proximity to the sun, since heat does not arise from the sun's nearness, but from the height and density of the atmosphere, as appears from the cold on high mountains even in hot climates; also that heat is varied according to the direct or oblique incidence of the sun's rays, as is plain from the seasons of winter and summer in every region.' It is curious to find thus advanced, in a sort of lecture addressed to visionary Mercurials, a theory which crops up repeatedly in the present day, because the difficulty which suggests it is dealt with so unsatisfactorily for the most part in our text-books of science. Continually we hear of some new paradoxist who propounds as a novel doctrine the teaching that the atmosphere, and not the sun, is the cause of heat. The mistake was excusable in Swedenborg's time. In fact it so chanced that, apart from the obvious fact on which the mistake is usually based--the continued presence, namely, of snow on the summits of high mountains even in the torrid zone--it had been shown shortly before by Newton, that the light fleecy clouds seen sometimes even in the hottest weather above the wool-pack or cumulus clouds are composed of minute crystals of ice. Seeing that these tiny crystals can exist under the direct rays of the sun in hot summer weather, many find it difficult to understand how those rays can of themselves have any heating power. Yet in reality the reasoning addressed by Swedenborg to his Mercurial friends was entirely erroneous. If he could have adventured as far forth into time as he did into space, and could have attended in the spirit the lectures of one John Tyndall, a spirit of our earth, he would have had this matter rightly explained to him. In reality the sun's heat is as effective directly at the summit of the highest mountain as at the sea-level. A thermometer exposed to the sun in the former position indicates indeed a slightly higher temperature than one similarly exposed to the sun (when at the same altitude) at the sea-level. But the air does not get warmed to the same degree, simply because, owing to its rarity and relative dryness, it fails to retain any portion of the heat which passes through it.