268 FKOM NEBULA TO NEBULA 



of the 24 hours and that it is a hemisphere, and not a flat 

 disc, with which we have to deal. Making corrections 

 for these facts, we find that, instead of onr former result 

 remaining at 46,000, it must be increased to 184,000 

 square feet, being equivalent to a small city block 425 

 feet each way, or, in the farmer 's standard, 4-J4 acres. 



Here we must call to mind that superimposed upon 

 this block is a great column of air, variously estimated 

 at from 50 to 150 miles in height, but attenuating rapidly 

 with the altitude. Not only, however, does the air de- 

 crease in density with altitude, but it decreases in tem- 

 perature as well ; and where the column may be supposed 

 to end, there is no confining wall, but the doorway is left 

 wide open, permitting the free escape of warmth into the 

 boundless outdoors of space, where the reigning tempera- 

 ture is generally believed to be absolute zero, or 273 C. 

 Inasmuch as it is impossible to make computations with- 

 out definite figures to begin with, let us try to approxi- 

 mate the facts, as nearly as we may, by assuming that the 

 atmospheric column in question is equivalent to another 

 column of air of sea-level density 7 miles in height and 

 possessed of an average temperature of C. It is such 

 a volume as this, then, 184,000 sq. ft. x 36,000 feet, that 

 modern scientists suppose to be maintained 273 C above 

 the absolute by the unaided radiation from a single 

 square foot of solar surface! We may emphasize these 

 figures by mentally dividing this great column into 

 smaller proportional dimensions. Imagine a hall 100 

 feet in the clear covering a ground space equivalent to a 

 small city block, then the contents will be 1-360 of our 

 column. Again, let it be assumed that the outside tem- 

 perature is 0C and that that within the hall is main- 

 tained at 27.3 C (the main doors being all the time kept 

 wide open) how large, think you, need the miniature sun 

 be to serve as the furnace? The cubical contents of the 

 hall being only 1-360 that of our atmospheric column, and 

 the lift in temperature only 1-10 as high, there would be 

 required a heat source possessing only 1-3600 of a square 

 foot of surface, or 1-5 inch square, indicating a sun no 



