Page:Astounding Science Fiction (1950-01).djvu/114

 315 West 33rd Street, New York, New York.

Dear Mr. Campbell:

Since you asked someone to check Art Kalaugher's figures, I accepted your challenge, and found that it was wrong. The correct value of π in the binary system is: 11.001001000011111101101010100010001000010110100011. If any of the readers of ASF have any use for it, I shall be glad to compute forty more places on it.

I enjoy Brass Tacks very much, especially the mathematical problems that some readers have. My only suggestion is that I think most of the editor's comments would be a lot better after the letter than before. And incidentally, are you convinced that you have a very good magazine, or do I have to repeat it too?—Gary D. Gordon, Wesleyan Station, Middletown, Connecticut.

Dear Mr. Campbell:

I should like to call attention to a slight error in an otherwise excellent article: that entitled "Parboiled Pilots" in the August, 1949, issue. On page 5 is stated : "Any means of expanding compressed air cools it...". (The italics are yours.)

It is a little-known fact that any gas upon expanding may either heat up or cool down, or even do both in the same operation, depending upon the nature of the gas, the temperature at which the operation started, and the extent of the expansion. Below a certain temperature called the Joule-Thomson inversion temperature—different for each gas: -83° C for hydrogen; much higher for most gases—gases do indeed cool down upon expanding, as you state, but above this temperature, astonishingly enough, they heat up. The attainment of the extremely low temperatures around absolute zero is dependent on the fact that liquid air can cool hydrogen below its inversion temperature, whereupon expansion can cool it further; the resulting liquid is then used for cooling helium below its inversion temperature. If air liquefied at a temperature above Hydrogen's JTIT, cryogenics would be crying indeed.

Further information may be found in any book of thermodynamic or advanced physical chemistry, e. g., S. Glasstone, "Chemical Thermodynamics," (D. Van Nostrand).

It may be splitting hairs, but you did use italics on the word "any"! I just want to set the record straight. Please accept my congratulations on the remainder of the article. 156