re suitable, in a strictly-scientific point of view, for
the foundations of a system, are less apparent, and require higher
powers of generalization and abstraction; and are, therefore, less in
accordance with the genius and spirit of the _Rollo_ philosophy.

As teachers have, in some cases, done the author the honor to introduce
some of the preceding works of this class into their schools, as reading
books, &c., considerable reference has been had to this, in the form and
manner of the discussion, and questions have been added to facilitate
the use of the books in cases where parents or teachers may make the
reading of them a regular exercise of instruction.

CONTENTS.

CHAPTER I. Page.
LOST IN THE SNOW, 9

CHAPTER II.
FLYING, 19

CHAPTER III.
VALVE MAKING, 40

CHAPTER IV.
EXPERIMENTS, 51

CHAPTER V.
PRESSURE, 64

CHAPTER VI
BALLOONING, 79

CHAPTER VII.
PHILOSOPHICAL DISCUSSION, 94

CHAPTER VIII.
TASKS, 108

CHAPTER IX.
BURNING, 121

CHAPTER X.
GRAVITATION, 143

CHAPTER XI.
AIR IN MOTION, 158

CHAPTER XII.
AIR AT REST, 178

CHAPTER I.

LOST IN THE SNOW.

One pleasant morning, very early in the spring, Rollo's cousin Lucy came
to call for Rollo to go on an expedition, which they had planned the day
before. It was near the end of March, and the snow had become so
consolidated by the warm sun in the days, and the hard frosts at night,
that it would bear the children to walk upon it. The children called it
the crust; but it was not, strictly speaking, a crust, for the snow was
compact and solid, not merely upon the top, but nearly throughout the
whole mass, down to the ground.

Rollo and Lucy were going to have an expedition upon the crust. Rollo
had a sled, and they were going to put upon the sled such things as they
should need, and Rollo was to draw it, while Lucy and Nathan, Rollo's
little brother, were to walk along by his side.

Rollo's sled was ready at the back door, when Lucy came. Lucy brought
with her some provisions for a luncheon, in a basket. This was her part
of the preparation. Rollo got his axe, and on