Take a balance, (see Plate,) DCE, fig. 4, the scales and arms of which are of such unequal weights and lengths, that the scale A may be in proportion to the scale B, as the length of the arm CE is to the length of the arm CD; then will the two scales be exactly in equilibrio about the point C; and the same will be the case, if the two arms CD, CE, are of equal length, but of unequal thickness, provided the thickness of CD is to that of CE, as the weight of the scale B is to that of A.
For example; suppose the arm CD is equal to three ounces, and the arm CE to two, and that the scale B weighs three ounces, and the scale A two; then the balance, in this case, will be exactly true when empty; and if a weight of two pounds be put into the scale A, and one of three pounds into B, they will still continue in equilibrio. But the fallacy in this, and all other cases of the same kind, may be easily detected, in shifting the weights from one scale to the other.
How to lift up a Bottle with a Straw, or any other slight Substance.
Take a straw, (see Plate,) AB, fig. 5, which is not broken or bruised, and bend one end of it into a sharp angle ABC; then if this end of the straw be put into the bottle, so that the bent part of it may rest against either of its sides, you may take the other end in your hand, and lift up the bottle by it without breaking the straw; and this will be the more easily done, according as the angular part of the straw approaches nearer to that which comes out of the bottle.
How to make a Cone, or Pyramid, move upon a Table without Springs, or any other artificial Means.
Take a cone, or pyramid, of paper, or any other light substance, and put a beetle, or some such small insect, privately under it; then, as the animal will naturally endeavour to free itself from its captivity, it will move the cone towards the edge of the table, and as soon as it comes there, will immediately return for fear of falling; and by moving backwards and forwards in this manner, will occasion much diversion to those who are ignorant of the cause.
To make a Pen, which holds One Hundred Sheep, hold double the Number, by only adding two Hurdles more.
In the first pen, or that which holds one hundred sheep, the hurdles must be so disposed, that there shall be only one at the top and bottom, and the rest in equal numbers on each side; then it is obvious, that if one hurdle more be placed at each end, the space enclosed must necessarily be double the former, and consequently will hold twice the number of sheep.
An ingenious Recreation, called the Two Communicative Busts.