A VERTICAL SECTION OF DAY OR SKY-LIGHT.

LET the arc ADCB represent the blue light of the atmospheric hemi-sphere or sky, the line AB a horizontal surface, ED a perpendicular surface, EC a diagonal surface:

Then, as a HORIZONTAL SURFACE is lighted by THE WHOLE ARC, a PER-PENDICULAR and a DIAGONAL by less than the whole;—therefore the UPPER SURFACES of bodies, usually assuming or inclining to the HORI-ZONTAL direction, will receive more of this blue light than their SIDES, which necessarily partake of the PERPENDICULAR or DIAGONAL;—and as we commonly say, the SUN RISES and the SUN SETS, and nobody quibbles about it, so the watchman of the DAY, as he tells the hours, may be permitted to add the highly necessary information, that

DAY-LIGHT SHINES DOWN PERPENDICULARLY.