CHAPTER I.. Q. _what Is The_ Principal _source of Heat_?

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.