CHAPTER I.

Q. _What is the_ PRINCIPAL _source of Heat_?

A. The SUN.

Q. _Why do_ BURNING GLASSES _set fire to substances submitted to their power_?

A. The rays of the sun, collected by the Burning Glass, are all _bent to one point_, called the "focus;" thus the heat and light, (which should be diffused over the _whole_ glass,) being gathered together into one point, are very greatly increased.

Q. _Why is there a_ DARK RIM _round this focus_?

A. Because the rays of light, which should have fallen there, are _bent into the focus_, and the space around, (being deprived of these rays) is accordingly darkened.

Q. _Are_ ALL _the rays bent into one point_?

A. No, not quite all: and, therefore, the rim round the focus is only _slightly_ shadowed.