Section 8. In construing any special act authorizing a city, town or district to incur indebtedness for a specified purpose to a limited amount, such limitation of amount shall be deemed to apply only to the indebtedness authorized by the special act, and not to affect any other power which the city, town or district may have to incur indebtedness for said purpose under any other provisions of law; and in special acts authorizing loans to be payable by proportionate payments, such provision shall be construed to mean payments as nearly equal in amount as is possible without unreasonable fractions, but no payment of the principal shall be greater than any preceding payment.