170 



SCIENCE 



[Vol. LVI, No. 1441 



find, when the sphere is as far from the sun as 

 the earth is, that the cap which faces the sun 

 will have a temperature of about 62° C. The 

 other cap will he at the absolute zero of tem- 

 perature, so that the average temperature of 

 the sphere will be about — 105° C. We may, 

 therefore, conclude that if a meteorite were 

 spherical and black its temperature when near 

 the earth would lie between — 105° C. and 

 +9° C. 



If the meteorite is not a perfect conductor 

 of heat the temperature of the inside will not 

 usually be the same as that of the outside. The 

 amount by which the temperature of the inside 

 differs from that of the outside will depend in 

 part on the thermal conductivity of the mete- 

 orite and in part on the rapidity of motion 

 toward the sun or away from it, as well as on 

 the distance from the sun. Suppose that the 

 meteorite is a sphere and has a diffusivity of 

 0.01 cm.^/sec. This is about the diffusivity 

 of limestone and is less than that of 

 granite. Suppose further that this sphere is 

 at the absolute zero of temperature and is sud- 

 denly placed where its surface is maintained at 

 9° C. Then for a sphere 10 cm. in diameter I 

 find- that at the end of fifteen minutes the dif- 

 ference between the temperature at the center 

 and that at the surface wiU be less than 15°, 

 at the end of twenty minutes the difference will 

 be less than 5°, and at the end of thirty min- 

 utes it will be less than 0.2°. If the meteorite 

 travels 100 kilometers a second it would re- 

 quire nearly five days to go a distance equal to 

 that from the or-bit of Venus to the orbit of 

 the earth. So, unless the meteorite travels 

 much faster than 100 kilometers a second, or is 

 much more than 10 cm. in diameter, it seems 

 likely that the temperature of the inside can 

 not be very different from that of the outside. 

 This, of course, is before the meteorite enters 

 the atmosphere of the earth. 



As another way of attempting to get some 

 idea as to the temperature of a meteorite sup- 

 pose we consider the temperatui'e of a cylinder. 

 Let the cylinder be at the same distance from 



- From eq. (44) on p. 133 of Ingersoll and 

 Zobel, The Mathematical Theory of Heat Con- 

 duction. 



the sun that the earth is. Let the ends of the 

 cylinder be black, and let one of them point 

 directly at the sun. To simplify matters Sup- 

 pose that there is no radiation from the sides 

 of the cylinder. Then the heat which reaches 

 the end that is turned toward the sun is partly 

 radiated from that end and partly conducted 

 to the other end and there radiated. From this 

 condition we obtain the equations 



'!i^\ + 6*) = b (3) 



and 



Mt 



Si = ©2 + -^> (4) 



where 6j and Oj stand for the absolute tempera- 

 tures of the two ends of the cylinder, and 

 I and k stand for the length and thermal con- 

 ductivity of the cylinder. Taking k as 0.008 

 cal./cm.sec.deg., which is about the value for 

 granite, equations (3) and (4) lead to the fol- 

 lowing results. 



Average 

 Length of Temperatures temperature 



cylinder of ends of cylinder 



1 cm. 61° C. 63° C. 62° C. 



10 cm. 52° C. 70° C. 61° C. 



100 cm. 4° C. 98° C. 51° C. 



In the actual case there would, of course, be 

 considerable radiation from the sides of the 

 cylinder, so that the temperatures would be 

 lower than those given in the above table. 



Although these calculations do not tell us 

 precisely what the temperature of a meteorite 

 may be expected to be just before it enters the 

 atmosphere of the earth, they do seem to be 

 sufficient to indicate that that temperature is 

 nearer to 0° C. than to the absolute zero of 

 temperature. 



Arthur Taber Jones 

 Smith College 



THE BUREAU OF STANDARDS 



To THE Editor op Science: Considering 

 the enormous interest taken even by the non- 

 discriminating newspaper public in the 

 achievements of the Bureau of Standards, 

 which scientific men are justly proud of as 

 one of the greatest physical laboratories of the 

 world, using physics in its proper sense of 

 including chemistry and engineerin"?, or ap- 



