Diameters.

Surfaces.

Cubical content

1

3.141

.523

2

12.567

4.188

3

28.274

14.137

4

50.265

33.51

5

78.54

65.45

10

314.159

523.6

15

706.9

1767.1

20

1256.6

4189

25

1963.5

8181

30

2827

14137

40

5026

33510

Owing to the increasing rarity of the atmosphere as we ascend from the earth's surface, balloon cases are made very much larger than is required to contain the necessary quantity of gas, to allow for its expansion as it rises into a rarer medium. A cubical foot of gas measured at the level of the sea, occupies a space of two feet at an elevation of 3½ miles. - Cooley's Cyclopedia.