114 



Dialogue between a Father and Son. 



Vol. IV. 



mon pump will draw or lift water more than 

 thirty-two feet in height. When you are 

 older, I will take you to see what are called 

 philosophical experiments; at present we must 

 be content to examine and reflect in a plain 

 way. 



Frank. — But I suppose that this might be 

 called a philo.'^ophical experiment? 



Father. — Yes — and with a very simple ap- 

 paratus too ; it is enough however to show you 

 the effect oF atmospheric pressure on the wa- 

 ter in a well. 



Frank. — Now, cannot you contrive to make 

 an air pump too ? 



Father. — Let us try — our tin cup will make 

 a decent substitute. Now till it with hot wa- 

 ter, and when it has stood awhile, empty it, 

 and turn it down into this saucer, and pour 

 a little water into it. 



Frank. — Oh ! how the bubbles are breaking 

 out all round the edge of the cup, and forcing 

 their way through the water in the saucer ! — 

 How is this ? 



Father. — The air, which is confined under 

 the cup, being rarefied or expanded by the 

 heat, forces its way round the edge of the 

 cup and through the water in bubbles. Now 

 wait a little, and take up the cup, and tell me 

 what you observe. 



Frank. — Well, I declare ! The saucer 

 sticks so closely to the cup as to be lifted up 

 by it ! How did you contrive this? 



Father. — It was no contrivance of mine — 

 it was caused by the pressure of the atmos- 

 phere: upon the escape of the expanded air 

 from the cup, none could enter to supply its 

 place because of the water in the saucer, 

 which formed what is called a water lute. — 

 On the coolmg of the cup, and consequent 

 contraction of that portion of air which re- 

 mained confined in it, a partial vacuum was 

 formed, when immediately the external air 

 being heavier than that which was contained 

 in the cup, pressed so forcibly on it as to keep 

 it fixed to tiie saucer. 



Frank. — This is indeed a capital air pump 

 — and I now see how it is, that when a piece 

 of leather, with a string fastened to it, is 

 stamped on a smooth stone, it sticks so closely 

 that the stone miglit be lifted by it : it is be- 

 cause the air has been driven out from under 

 the leather, and then the pressure of the at- 

 mosphere falls upon it and fixes it to the 

 stone. 



Father. — It is so, and that you may per- 

 ceive how we are surrounded vvitli objects 

 which call for observation, I will mention a 

 very tamiliar example of the pressure of the 

 atmosphere, the effects of which caimot have 

 escaped your notice. You know that when 

 we cross the lane leading to Farmer Vince's 

 house, we arc in danger of losing our shoes 

 by sticking in the clay ! 



Frank'. — Yes — 1 have often done that, I 

 know. 



Father. — The clay being soil, takes so ex- 

 act an impression of the shoe, that at every 

 step the air is driven out from under the foot 

 by the pressure of the atmosphere, and the 

 foot is so forcibly pinned down that it is often 

 difficult to raise it. And what is it, do you 

 think, which presses so heavily the hot iron 

 with which your mother smoothes the clothes 

 after washing? The air is expanded by the 

 heat, and is driven out fi-om under it, when, 

 immediately, the external pressure on the 

 iron, great in proportion to its heat, is suffi- 

 cient to answer the purpose of a mangle 

 heavily loaded. 



Frank. — Our theory of haymaking, I de- 

 clare ! 



Father. — Exactly — and I am pleased with 

 your application. 1 was going to say, take a 

 cold iron, and rub it with all your might, and 

 you will scarcely make any impression — the 

 same iron made hot gives one the idea of 

 copperplate pressure ; and this is increased 

 by damping the clothes, as by that means, the 

 expansion of the air is greater, and the vacu- 

 um thus created more perfect. And see, how 

 one observation leads to another. You have 

 heard London dirt spoken of by those who 

 have visited that place, and they declare there 

 is no walking the streets without splashing 

 one's self from head to foot. The reason is 

 this — the pavement is worn so smooth, that at 

 every step the shoe fits it so closely, that tlie 

 air is driven forcibly out from under it ; and 

 then the foot falls so heavily in consequence 

 of the pressure of the atmosphere upon it, 

 that the mud is scattered about on all sides, 

 as though it were hloicn up; while here, in 

 the countr}', we can walk in the worst roads 

 without scattering the dirt, as the inequalities 

 in the ground give space for the escape of 

 the air, and support the pressure of the foot, 

 by which all splashing is prevented. 



Frank. — I wonder how many of the thou- 

 sands who crowd tlie streets of Jjondon, a!ul 

 complain of the mud with which they are 

 covered, ever think of attributing it to the 

 right cause, — the pressure of the atmosphere? 



Father. — There is just one more instance 

 which I will mention. You iiave often heard 

 persons, wiio wear India rubber shoes, com- 

 plain of the pain which they experience, if 

 they keep them long on their feet, especially 

 in warm rooms. This is occasioned hy the 

 pressure of the atmosphere upon the fi.'et, in 

 consequence of the vacuum that has been 

 formed around the feet by the tigiitness of the 

 rubber shoes, which fit so closely as to prevent 

 any air from entering to supply the i)lace of 

 that which has been forced out hy expansion, 

 caused hy the heat of the feet: evaporation 

 can therefore no longer go en, and the stock- 



