Conversations on Natural Philosophy, in which the Elements of that Science are Familiarly Explained Part 35

_Emily._ Water cannot, then, be raised by a pump, above 32 feet; for the pressure of the atmosphere will not sustain a column of water, above that height.

_Mrs. B._ I beg your pardon. It is true that there must never be so great a distance as 32 feet, from the level of the water in the well, to the valve in the piston, otherwise the water would not rise through that valve; but when once the water has pa.s.sed that opening, it is no longer the pressure of air on the reservoir, which makes it ascend; it is raised by lifting it up, as you would raise it in a bucket, of which the piston formed the bottom. This common pump is, therefore, called the sucking, or lifting pump, as it is constructed on both these principles.

The rod to which the piston is attached, must be made sufficiently long, to allow the piston to be within 32 feet of the surface of the water in the well, however deep it may be. There is another sort of pump, called the forcing pump: it consists of a forcing power, added to the sucking part of the pump. This additional power, is exactly on the principle of the syringe: by raising the piston, you draw the water into the pump, and by causing it to descend, you force the water out.

_Caroline._ But the water must be forced out at the upper part of the pump; and I cannot conceive how that can be done by the descent of the piston.

_Mrs. B._ Figure 5, plate 14, will explain the difficulty. The large pipe, A B, represents the sucking part of the pump, which differs from the lifting pump, only in its piston P, being unfurnished with a valve, in consequence of which the water cannot rise above it. When, therefore, the piston descends, it shuts the valve Y, and forces the water (which has no other vent) into the pipe D: this is likewise furnished with a valve V, which, opening upwards, admits the water to pa.s.s, but prevents its return.

The water, is thus first raised in the pump, and then forced into the pipe, by the alternate ascending, and descending motion of the piston, after a few strokes of the handle to fill the pipe, from whence the water issues at the spout.

_Emily._ Does not the air pump, which you used in the experiments, on pneumatics, operate upon the same principles as the sucking pump?

_Mrs. B._ Exactly. The air pump which I used (plate 1, fig. 2,) has two hollow, bra.s.s cylinders, called barrels, which are made perfectly true.

In each of those barrels, there is a piston; these are worked up, and down, by the same handle; the pistons, are furnished with valves, opening upwards, like those of the common pump: there are valves also, placed at the lower part of each barrel, which open upwards; there are therefore two pumps, united to produce the same effect: two tubes, connect these barrels with the plate, upon which I placed the receivers, which were to be exhausted.

_Emily._ I now understand how the air pump acts; the receiver contains air, which is exhausted, just as it is by the common pump, before the water begins to rise.

_Mrs. B._ Having explained the mechanical properties of air, I think it is now time to conclude our lesson. When next we meet, I shall give you some account of wind, and of sound, which will terminate our observations on elastic fluids.

_Caroline._ And I shall run into the garden, to have the pleasure of pumping, now that I understand the construction of a pump.

_Mrs. B._ And, to-morrow, I hope you will be able to tell me, whether it is a forcing, or a common lifting pump.

Questions

1. (Pg. 136) Into what two kinds are fluids divided?

2. (Pg. 136) There are different kinds of elastic fluids, in what properties are they alike, and in what do they differ?

3. (Pg. 136) In what particular do elastic, differ from non-elastic, fluids?

4. (Pg. 136) What is meant by the elasticity of air?

5. (Pg. 137) What is said respecting the weight of the atmosphere?

6. (Pg. 137) Why do we not feel the pressure of the air?

7. (Pg. 137) What would be the effect of relieving us from atmospheric pressure?

8. (Pg. 138) How may the weight of the air be shown by the aid of the air pump, and a piece of bladder?

9. (Pg. 138) How is this explained?

10. (Pg. 138) How may its elasticity be exhibited, by an apple, and by a bladder?

11. (Pg. 139) What is the absolute weight of a given column of atmospheric air, and how could its whole pressure upon the earth be ascertained?

12. (Pg. 139) How can the weight of a small bulk of air be found?

13. (Pg. 140) In ascertaining the weight of air, we take account of its temperature--Why?

14. (Pg. 140) How could you ascertain the specific gravity of air, and what would it be?

15. (Pg. 140) What are the essential parts of a barometer, as represented plate 14, fig. 3?

16. (Pg. 141) What sustains the mercury in the tube?

17. (Pg. 141) Of what use are the divisions in the upper part of the instrument?

18. (Pg. 141) To what height will the mercury rise, and what occasions this height to vary?

19. (Pg. 141) When is the mercury highest, in wet, or in dry weather?

20. (Pg. 141) What occasions the sensation of oppression, in damp weather?

21. (Pg. 142) Why will the barometer indicate the height of mountains, or of balloons?

22. (Pg. 142) Is any inconvenience experienced by persons ascending to great heights, and from what cause?

23. (Pg. 142) What occasions the rise and fall of the mercury, in a thermometer?

24. (Pg. 142) To what height will the pressure of the atmosphere raise a column of water?

25. (Pg. 142) What governs the difference between the height of the mercury, and of the water?

26. (Pg. 143) How does the common pump, raise water from a well?

27. (Pg. 143) What is meant by a piston?

28. (Pg. 143) Describe the construction, and use, of a valve.

29. (Pg. 143) What are the parts of the pump, as represented, fig. 4, plate 14.?

30. (Pg. 144) How do these parts act, in raising the water?

31. (Pg. 144) In what does that which is commonly called suction, consist?

32. (Pg. 144) How must the piston be situated in the pump?

33. (Pg. 144) What other kind of pump is described?

34. (Pg. 145) How is the forcing pump constructed, as shown in plate 14, fig. 5?

35. (Pg. 145) Describe the construction and operation of the air pump, (fig. 2, plate 1.)