Head, Justice.
1. “Powers of all public officers are defined by law, and all persons must take notice thereof. The public may not be estopped by the acts of any officer done in the exercise of a power not conferred.” Code, § 89-903.
2. County boards of tax assessors are limited by law in the employment of agents. Such agents may be employed to seek out unreturned property for taxation. They may be paid a commission not exceeding 10% of the county and school tax on returned property placed on the digest “by the efforts of said agents.” Code (Ann.), Chapter 92-69, § 92-6910 (Ga. L. 1913, p. 123, as amended, Ga. L. 1937, p. 517). The Board of Tax Assessors of Hall County can not delegate its duties pertaining to the equalization of property valuations for ad valorem taxation, nor can the board contract for services of the nature stipulated in the contract executed by the board. No. 17110.
July 11, 1950.
Rehearing denied July 24, 1950.
3. The Board of Commissioners of Roads and Revenues of Hall County has neither express nor implied authority to contract with reference to any of the powers and duties of the Board of Tax Assessors of Hall County. Ga. L. 1935, p. 661; Decatur County v. Roberts, 159. Ga. 528 (126 S. E. 460).
4. Constitutional questions will not be decided by this court unless a ruling thereon is essential in the determination of the cause.
5. The trial court did not err in overruling the demurrers to the petition.
Judgment affirmed.
All the Justices concur. Duckworth, C.J., concurs specially.
A. R. Kenyon and Kenyon, Kenyon & Gunter, for plaintiffs in error.
Stow & Royal and G. Fred Kelley, contra.
Emory F. Robinson, amicus curiae.