gravity? We have been discussing that matter this afternoon."

"Gravity is a force by virtue of which all bodies tend to approach each
other constantly; and weight is the measure of the effect of gravity
acting upon a body. The two are often confounded."

"Now, the next thing we want to know is, does gravity act in all
directions?"

"Yes; and the attraction of one body for another is in proportion to its
mass--that is to say, if two bodies, one weighing one and the other two
pounds, should be hung near each other, the heavy object would pull the
lighter one twice the distance it would itself move."

"Do you mean to say that everything on earth attracts everything else? I
thought it was only the earth that had the power to attract."

"The earth is no different from any other object in that particular, but
on account of its immense size everything goes toward it and its motion
toward the small object is not noticed."

"Is there any way that such a statement could be proved?"

"I suggest one plan: select two walls, close together, or two tall
trees, and run a wire across, as I show in the sketch (Fig. 32). From
that cross wire, A, suspend three objects by cords, B, C, D. The cord B
is exactly midway between the two walls, and the other cords C, D, and
so attached that the objects at their lower ends hang close to the
walls. It will be found that the cords C, D are farther apart at their
lower ends than at the upper ends, and that the cord B is exactly plumb,
as it is affected equally by the attraction of the opposite walls."

[Illustration: _Fig. 34. Gravitational pull._]

The new raft attachment was completed, and Harry made the suggestion
that it ought to be put to a practical test, and that a good place to do
this would be below the cataract, where it was wide and deep enough to
float.

The drill was affected for attaching the sections, as shown in the
drawing (Fig. 35), in which A represents the section under the axles,
and B, C the two side sections, attached at their inner edges to the
ends of the axles, and with the posts D serving as braces.

The yaks seemed to know their business instinctively, and moved down
into the water slowly, and the improvised raft not only prevented the
body from sinking into the water very low, but it had a wonderful
steadying effect, because the side sections served as wings to prevent
lateral swaying.

[Illustration: _Fig. 35. Using the Fort as a Raft._]

The crossing an