NEWS FROM JUPITER. 435 



semblance between Jupiter and our earth, we may safely (so far as our 

 inquiry is concerned) proceed on the assumption that the atmosphere 

 of Jupiter does not differ greatly in constitution from that of our 

 earth. We may further assume that, at the upper part of the cloud- 

 layers we see, the atmospheric pressure is not inferior to that of our 

 atmosphere at a height of seven miles above the sea-level, or one-fourth 

 of the pressure at our sea-level. Combining these assumptions with 

 the conclusion just mentioned, that the cloud-layers are at least 100 

 miles in depth, we are led to the following singular result as to the 

 pressure of the Jovian atmosphere at the bottom of the cloud-layer : 

 The atmosphere of any planet doubles in pressure with descent through 

 equal distances, these distances depending on the power of gravity at 

 the planet's surface. In the case of our earth, the pressure is doubled 

 with descent through about 3|- miles ; but gravity on Jupiter is more 

 than 2£ times as great as gravity on our earth, and descent through 

 If mile would double the pressure in the case of a Jovian atmosphere. 

 Now, 100 miles contain this distance (If mile) more than seventy-one 

 times ; and we must therefore double the pressure at the upper part 

 of the cloud-layer seventy-one successive times to obtain the pressure 

 at the lower part. Two doublings raise the pressure to that at our sea- 

 level ; and the remaining sixty-nine doublings would result in a press- 

 ure exceeding that at our sea-level so many times that the number 

 representing the proportion contains twenty-one figures. 1 I say would 

 result in such a pressure, because in reality there are limits beyond 

 which atmospheric pressure cannot be increased without changing the 

 compressed air into the liquid form. What those limits are we do not 

 know, for no pressure yet applied has changed common air, or either 

 of its chief constituent gases, into the liquid form, or even produced 

 any. trace of a tendency to assume that form. But it is easily 

 shown that there must be a limit to the increase of pressure which air 

 will sustain without liquefying. For the density of any gas changes 

 proportionately to. the increase of pressure until the gas is approaching 

 the state when it is about to turn liquid. Now, air at the sea-level has 

 a density equal to less than the 900th part of the density of water ; so 

 that, if the pressure at the sea-level were increased 900 times, either 

 the density would not increase proportionally, which would show that 

 the gas was approaching the density of liquefaction, or else the gas 

 would be denser than water, which must be regarded as utterly impos- 



1 The problem is like the well-known one relating to the price of a horse, where 

 one farthing was to be paid for the first nail of 24 in the shoes, a half-penny for the 

 next, a penny for the third, two pence for the fourth, and so on. It may be inter- 

 esting to some of my readers to learn, that if we want to know roughly the proportion 

 in which the first number is increased by any given number of doublings, we have 

 only to multiply the number of doublings by Voths, and add 1 to the integral part of the 

 result, to give the number of digits in the number representing the required proportions. 

 Thus multiplying 24 by i 3 -ths gives 7 (neglecting fractions) ; and therefore the number 

 of farthings in the horse problem is represented by an array of 8 digits. 



