Section 30. A petition under either of the two preceding sections shall set forth a description of the estate to be mortgaged, the amount of money necessary to be raised and the purposes for which it is required; and the decree of the court upon such petition shall fix the amount for which the mortgage may be given and the rate of interest which may be paid thereon, and may order the whole or any part of the money secured by the mortgage to be paid from time to time out of the income of the property mortgaged.