After the return of the assessment roll to the county legislative authority it shall make an order setting a day for the hearing upon any objections to the assessment roll by any parties affected thereby who shall be heard by the county legislative authority as a board of equalization, which date shall be at least twenty days after the filing of such roll. It shall be the duty of the county legislative authority to give, or cause to be given, notice of such assessment, and of the day fixed for the hearing, as follows:

(1) They shall send or cause to be sent, by mail, to each owner of premises assessed, whose name and place of residence is known to them, a notice, substantially in this form, to wit:

". . . .

"Your property (here describe the property) is assessed $. . . . . . for river and harbor improvement to be made in this county.

"Hearing on the assessment roll will be had before the undersigned, at the office of the county commissioners, on the . . . . day of . . . . . . (year) . . . .

But failure to send, or cause to be sent, such notice, shall not be fatal to the proceedings herein prescribed.

(2) They shall cause at least ten days' notice of the hearing to be given by posting notice in at least ten public places in the county, three of which shall be in the neighborhood of the proposed improvement, and by publishing the same at least once a week for two consecutive weeks in the official newspaper of the county which notice shall be signed by the county legislative authority, and shall state the day and place of the hearing of objections to the assessment roll, and the nature of the improvement, and that all interested parties will be heard as to any objections to said assessment roll.