Volume 1

Lectures on select subjects in mechanics, hydrostatics, hydraulics, pneumatics, optics, geography, astronomy, and dialling / By James Ferguson. With notes, and an additional volume, containing the most recent discoveries in the arts and sciences. By David Brewster.

  • Ferguson, James, 1710-1776.
Date:
1823
Catalogue details

Licence: Public Domain Mark

Credit: Lectures on select subjects in mechanics, hydrostatics, hydraulics, pneumatics, optics, geography, astronomy, and dialling / By James Ferguson. With notes, and an additional volume, containing the most recent discoveries in the arts and sciences. By David Brewster. Source: Wellcome Collection.

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    3. If two drops of quicksilver be placed near each other, they will run together, and become one large drop. 4. If two pieces of lead be scraped clean, and pressed toge- ther with a twist, they will attract each other so strongly, as to require a force much greater than their own weight to separate them. And this cannot be owing to the pressure of the air, for the same thing will hold in an exhausted receiver. 5. If two polished plates of marble or brass be put together, with a little oil between them, to fill up the pores in their sur- faces, and prevent the lodgment of any air, they will cohere so strongly, even if suspended in an exhausted receiver, that the weight of the lower plate will not be able to separate it from the upper one. In putting these plates together, the one should be rubbed upon the other, as a joiner does two pieces of wood when he glues them.6 6. If two pieces of cork, equal in weight, be put near each other in a bason of water, they will move equally fast toward each other with an accelerated motion, until they meet; and then, if either of them be moved, it will draw the other after it. If two corks of unequal weights be placed near each other, they will approach with accelerated velocities inversely propor- tional7 to their weights ; that is, the lighter cork will move as i 6 The experiment thus stated does not properly exemplify the attraction of co- hesion ; for when oil, or any other fluid, is interposed between the plates of marble, it may reasonably be conjectured that the cohesion of the surfaces arises from the viscidity of the intermediate substance. Those who have been in the habit of con- structing metals for reflecting telescopes, may probably have seen a striking exem- plification of this species of attraction. We have repeatedly ground together a con- vex and a concave brass tool, with such accuracy, that the one would raise the other from the ground without the interposition of any liquid, though its weight was about two pounds, and the contiguous surfaces completely polished. When the tools of brass are small, the oneuvill raise the other, even when a fibre of silk is placed be- tween their surfaces.—Ed. 7 When two variable or changeable quantities, A and B, are so related to each other, that when the one increases or diminishes, the other increases or diminishes in the same proportion, A is said to be directly proportional to B. Thus, if A represents the attraction of any planet, and B its quantity of mat- ter, A is directly proportional to B ; because if the quantity of matter is doubled, the force of attraction is also doubled, and vice versa. If when B increases, A diminishes, or when B diminishes A increases, A is then said to be in- versely proportional to B. If A increases as the square of B increases, A is said to be directly proportional to the square of B. If A increases as the square of B diminishes, A is said to be inversely proportional to the square of B, or inverse*