i88o.] 
On Water and Air * 
517 
above the water. Now, having forced sufficient into the 
fountain, I will shut the stopcock and remove the syringe. 
Many fountains play upon this principle, but it is simply an 
illustration of the spring of air which was so completely 
investigated by Robert Boyle. At the present time you have 
compression of the air by means of this spring to which 
Robert Boyle refers. This spring of the air which has been 
forced upon the surface of the water will drive out the water. 
I will now open the stopcock, and you see that the water is 
ejected through the tube ef to a considerable height, and 
continues to play until the vessel is emptied. 
There is another experiment referred to by Boyle to which 
I will draw your attention. Here are two plates of marble 
that have been in this institution for fifty or sixty years ; at 
all events they were here long before my arrival, and they 
were obviously made for the purpose of illustrating the 
experiment of Boyle. We take these two plates of marble, 
which are intended to be very smooth, and slide their flat 
surfaces over each other : there is a certain definite way of 
causing them to cling together. I feel the clinging force 
very much. They ought to cling and hold fast together. 
Now Boyle entertained the idea — and the idea has come 
down to the present time— that when you pass one of these 
plates over the other, and cause them to come into close 
contact with each other, you squeeze out the air, and hence 
the atmospheric pressure outside comes into play and keeps 
the plates of marble together. I am indebted to one of the 
greatest mechanical geniuses of our age, Sir Joseph Whit- 
worth, for the two metal planes that are now before you. 
I want to show you their action. These two planes of iron 
were produced by Sir Joseph Whitworth, who has done this 
in a manner that really is perfectly wonderful. He has 
produced what are called true planes. He started in life 
with the idea of making mechanism as perfect as possible, 
and his first idea was to obtain perfectly flat surfaces. I 
wish I had time to go fully into the manner in which he has 
done it. He has done it in the simplest manner possible, 
by first of all making them approximate to smooth sui faces. 
He then placed a coating of rouge on one of the surfaces, 
and, placing this upon the other surface, pressed them 
together. If they were perfectly smooth the rouge would 
cover both surfaces uniformly ; but there are always little 
hollows, and hills, and eminences, which left the louge in 
spots and patches, and by scraping away these eminences 
lie arrives at his true planes. When I place the two to- 
gether there is a most extraordinary effect. The upper one 
