350 
POPULAK SCIENCE REVIEW. 
question whether there can be any resemblance 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-i 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 pressure 
exceeding that at our sea-level so many times that the number 
representing the proportion contains twenty-one figures.* I 
say would result in such a pressure, because in reality there 
are limits beyond which atmospheric pressure cannot be in- 
creased 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 
* The proWem 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 
halfpenny for the next, a penny for the third, two pence for the fourth, and 
so on. It may be interesting 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 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 jo"*^hs gives 7 (neglecting fractions) ; and therefore 
the number of farthings in the horse problem is represented by an array of 
8 digits. 
