Coal was an essential source of fuel at that time.

Productivity is in this economic system the most important feature and an essential source of incomes.

The Hall has continued to receive funding from a number of sources essential to its operation and expansion.

They are an essential source for scholars who study the period.

However, this book remains an essential source on the life and times of the author.

Despised, rejected, yet needed as an essential source of labor.

New York officials have argued that the 24-mile, 330-megawatt cable is an essential source of extra energy.

This subsidy is an essential source of income for the counties.

It has been used as an essential source to fund rural development schemes throughout the country.

Ultimately, however, her commitment to survival arose from another, more essential source.