_Caroline._ That is to say, by converting the rolling friction into the rubbing friction. And when you had casters put to the legs of the table, in order to move it more easily, you changed the rubbing into the rolling friction.

_Mrs. B._ There is another circ.u.mstance which we have already noticed, as diminishing the motion of bodies, and which greatly affects the power of machines. This is the resistance of the medium, in which a machine is worked. All fluids, whether elastic like air, or non-elastic like water and other liquids, are called mediums; and their resistance is proportioned to their density; for the more matter a body contains, the greater the resistance it will oppose to the motion of another body striking against it.

_Emily._ It would then be much more difficult to work a machine under water than in the air?

_Mrs. B._ Certainly, if a machine could be worked in _vacuo_, and without friction, it would not be impeded, but this is unattainable; a considerable reduction of power must therefore be allowed for, from friction and the resistance of the medium.

We shall here conclude our observations on the mechanical powers. At our next meeting I shall endeavour to give you an explanation of the motion of the heavenly bodies.

Questions

31. (Pg. 62) Describe a pulley, and its use.

32. (Pg. 62) What is meant by a fixed pulley and why is not power gained by its employment? (fig. 1. plate 5.)

33. (Pg. 62) Of what use is the fixed pulley?

34. (Pg. 63) How is the power gained by a moveable pulley, explained by means of fig. 2. plate 5?

35. (Pg. 63) What proportion must the power bear to the weight in fig.

2, that their momentums may be equal?

36. (Pg. 64) What is a fundamental law as respects power and time?

37. (Pg. 64) If to gain power we must lose time, what advantage do we derive from the mechanical powers?

38. (Pg. 64) What name is given to two or more pulleys connected by one string?

39. (Pg. 64) How do we estimate the power gained by a system of pulleys?

40. (Pg. 65) What is represented by fig. 5. plate 5?

41. (Pg. 65) How does the wheel operate in increasing power?

42. (Pg. 65) How is this compared with the lever?

43. (Pg. 65) How does a handle fixed to an axle, represent a wheel, fig.

6?

44. (Pg. 65) How could we increase the power in this instrument?

45. (Pg. 66) What other forces besides the power of men, do we employ to move machines?

46. (Pg. 66) What will serve as an example of an inclined plane?

47. (Pg. 66) In what proportion does it gain power? (fig. 7.)

48. (Pg. 66) To what is the wedge compared? (fig. 8.)

49. (Pg. 66) How does its power increase?

50. (Pg. 67) Why is it rather a compound than a simple power?

51. (Pg. 67) What common instruments act upon the principle of the inclined plane, or the wedge?

52. (Pg. 67) Why does a knife cut best when drawn across?

53. (Pg. 67) The screw has two essential parts; what are they?

54. (Pg. 67) What other instrument is used to turn the screw?

55. (Pg. 67) How can you compare the screw with an inclined plane? Fig.

10.

56. (Pg. 68) By what two means may the power of the screw be increased?

57. (Pg. 68) How do we estimate the power gained by the screw?

58. (Pg. 68) Is the lever always attached to the nut, as in the figure?

59. (Pg. 68) What is said respecting the composition of all machines, and for what must allowance always be made in estimating their power?

60. (Pg. 69) What is meant by friction, and what causes it?

61. (Pg. 69) How may friction be diminished?

62. (Pg. 69) Friction is of two kinds, what are they?

63. (Pg. 69) For what purpose are wheels often used?

64. (Pg. 69) When is the friction of a carriage wheel changed from the rolling to the rubbing friction?

65. (Pg. 70) What is a medium, and in what proportion does it diminish motion?

66. (Pg. 70) Under what circ.u.mstances must a body be placed, in order to move without impediment?

CONVERSATION VI.

CAUSES OF THE MOTION OF THE HEAVENLY BODIES.

OF THE EARTH'S ANNUAL MOTION. OF THE PLANETS AND THEIR MOTION. OF THE DIURNAL MOTION OF THE EARTH AND PLANETS.

CAROLINE.

I am come to you to-day quite elated with the spirit of opposition, Mrs.