(1) In case the directors or commissioners of any improvement district find it desirable to do so, they may issue coupon bonds or other negotiable evidences of indebtedness for the purpose of raising money to pay preliminary expenses.
(2) These bonds or other evidences of indebtedness may be issued, from time to time, in payment for preliminary work done. The directors or commissioners may secure the payment of the bonds by a pledge and mortgage of the assessment of benefits whenever the assessment of benefits has been confirmed.