4. It is the safest.
5. It is inspected without difficulty.
6. It is most symmetrical.
7. It is manufactured easier.
8. It resists internal strain better.
9. It resists external strain also.
10. It can be stayed or strengthened better.
11. It encloses the greatest volume with least material.
12. It is the result of many years’ experience in boiler practice.
13. It is the form adopted or preferred by all experienced engineers.