On Water and Air . 
329 
1880.] 
a kind of limit to the height of the atmosphere, but all give 
it a certain height ; some have held it to be about fifty 
miles in height, others one hundred, others two hundred ; 
but, at all events, whatever the height is these lower strata 
of the atmosphere have to bear the weight of all the atmo- 
sphere above them. 
Now, upon my body at the present moment the entire 
weight of the atmosphere is pressing, and I wish you to 
clearly realise that this pressure is not only exerted upon 
my head, which has to bear all the weight of the atmosphere 
above it, but that it is just as much exerted upon my sides ; 
so that when you calculate the pressure exerted upon the 
human body, you must take the number of square inches 
upon the whole of the human body, and you must determine 
the pressure upon every square inch. What is that pres- 
sure ? The pressure upon every square inch is exadfly 
equal to that bar of lead, which is a square inch thick and a 
yard in height. Now, how would you like to bear the weigh 
of a mass of lead surrounding your body? Well, what does 
that amount to in its totality ? It amounts to a pressure of 
no less than 14 pounds. Every man here bears a pressure 
of this kind — a pressure of 14 pounds upon every square 
inch of his body. The reason why we are not crushed by 
this pressure is that the liquids of our bodies are nearly in- 
compressible. They are like water, and the air within our 
bodies is just as much compressed as the air without, so 
that there is really no difficulty in existing under these cir- 
cumstances. Well, things went on in this way until one 
day, as it is said, some gardeners wanted to water their 
garden in Florence, and they came to Galileo, and told him 
that they found that the water refused to rise above a cer- 
tain level in the pump. I will not vouch for the perfect 
accuracy of this account, but I think that it is very likely 
that it is correct. Here is a point worth noting. Great 
inventions are sometimes made before the reasons for them 
are known. In this way gunpowder was invented. The 
desire of man to kill his fellow man was so very strong and 
strenuous that it was the motive power to induce him to 
select the constituents of gunpowder in their proper propor- 
tions, although they knew not one iota about the chemistiy 
of the thing. The chemist came afterwards. But it was 
not until afterwards that he explained that these men were 
correct, and that the proportions were remarkably just. 
And thus, as I have said, great inventions are made before 
the scientific reasons for these inventions are known. And 
thus it was with this discovery of the pump. 
