Scientific Lectures. 57 



importance of all that refers to the atmosphere. Under the term 

 " climate " we include the heat of the atmosphere, the amount 

 of water which it usually contains in the shape of invisible vapor, 

 and last of all, the weight or density of the atmosphere. These 

 are the three great elements of investigation which we have to con- 

 sider in the discussion of climate. But our subject is pre-eminently 

 (though it cannot be entirely) confined to the investigation of the 

 weight of the atmosphere by means of the instrument which 

 we now see before us — the barometer. The atmosphere is a very 

 simple garment around the earth, and a very thin one. Its total 

 height, as far as it can be estimated is no more than forty-five 

 miles; and, owing to its extreme compressibility, three or four 

 miles of the atmosphere, that is above the level of the sea, 

 contains more than half the total weight ; and it is in that very 

 thin layer that the whole of the life of the globe is found and per- 

 forms its functions ; and what is very important, after all, in this 

 large globe of ours, is what is going on within that very thin cuti- 

 cle of atmosphere. It is very important to see what is taking place 

 in that part, and we come now to the subject of the barometer. This 

 word means " an instrument to measure weight " — two Greek words 

 signifying to measure weight — but until the seventeenth century it 

 was scarcely believed, except by a few select minds, that the atmos- 

 phere, which is composed of repellant atoms, could be weighed at all, 

 Aristotle tried whether there was any weight in it, but his experi- 

 ment, not being well managed, gave no result, and until the 17th 

 century it was thought, though nature has a horror of a vacuum, the 

 air had no weight at all. It is due to Galileo to mention that he 

 discovered the fact that air has weight ; by this means he explained 

 the circumstance that water will not rise in wells beyond a certain 

 limit, and explained the phenomena of suction ; and he was in the 

 midst of those experiments when, old and broken down by persecu- 

 tion, he died. He left it to Torricelli to discover the barometer. He 

 thought of a simple experiment to demonstrate the weight of the 

 atmosphere. He thought at first of filling a tube with water ; but after- 

 wards he filled a tube with mercury (which is thirteen and a-half 

 times heavier than water) and then turning this tube into a cup of 

 mercury, lo ! it was found that the mercury would not sink down 

 out of the tube, but that it stopped at about thirty inches above the 

 level of the sea — and then was the barometer invented. The whole 

 weight of the atmosphere would be expressed by the column of 



