<PAGE>

                                                                     Exhibit 4.7

________________________________________________________________________________


                                BY AND BETWEEN

                          THE PROVIDENCE GAS COMPANY


                                      AND


                     STATE STREET BANK AND TRUST COMPANY,
          as Successor to Rhode Island Hospital Trust National Bank,
             as Successor to Rhode Island Hospital Trust Company,
                                    Trustee



                      Twenty-First Supplemental Indenture



________________________________________________________________________________
<PAGE>

     THIS TWENTY-FIRST SUPPLEMENTAL INDENTURE, dated as of September 1, 1999, is
between The Providence Gas Company, a corporation created by Special Act of the
General Assembly of the State of Rhode Island (together with its successors and
assigns, the "Company" or "Providence"), and State Street Bank and Trust
Company, a national banking association having offices in Providence, Rhode
Island, as successor to Rhode Island Hospital Trust National Bank as Successor
to Rhode Island Hospital Trust Company, as Trustee under the Company's First
Mortgage described below (together with its successors and assigns, the
"Trustee").

     WHEREAS the Company, by an Indenture of First Mortgage, dated as of January
1, 1922 (the "Original Indenture"), as supplemented by a First Supplemental
Indenture dated as of February 6, 1933, a Second Supplemental Indenture dated as
of June 1, 1944, a Third Supplemental Indenture dated as of April 1, 1948, a
Fourth Supplemental Indenture dated as of January 1, 1958, a Fifth Supplemental
Indenture dated as of July 1, 1960, a Sixth Supplemental Indenture dated as of
September 1, 1963, a Seventh Supplemental Indenture dated as of December 1,
1971, the Eighth Supplemental Indenture dated as of July 1, 1972, the Ninth
Supplemental Indenture dated as of October 1, 1975, the Tenth Supplemental
Indenture dated as of April 1, 1976, the Eleventh Supplemental Indenture dated
as of September 1, 1978, the Twelfth Supplemental Indenture dated as of
September 1, 1983, the Thirteenth Supplemental Indenture dated as of May 1,
1986, the Fourteenth Supplemental Indenture dated as of August 1, 1988, the
Fifteenth Supplemental Indenture dated as of June 1, 1990, the Sixteenth
Supplemental Indenture dated as of September 1, 1992, the Seventeenth
Supplemental Indenture dated as of November 1, 1993, the Eighteenth Supplemental
Indenture dated as of December 1, 1995, the Nineteenth Supplemental Indenture
dated as of April 1, 1998, and the Twentieth Supplemental Indenture dated as of
February 1, 1999 (the Original Indenture as supplemented by each such
supplement, including, if appropriate, this Twenty-First Supplemental Indenture,
and by any additional supplements as shall have been executed and delivered at
the pertinent time, the "Indenture"), mortgaged its property and franchises,
including after-acquired property and franchises, to the Trustee to secure the
First Mortgage Bonds issued and to be issued thereunder in accordance with the
provisions of said Indenture or First Mortgage Bonds, as heretofore
supplemented, and there are now outstanding thereunder $1,819,000 principal
amount of First Mortgage bonds, Series M, 10.25% due July 31, 2008 (being part
of an original issue of $10,000,000 in principal amount of such Series M bonds);
$10,000,000 principal amount of First
<PAGE>

Mortgage bonds, Series N, 9.63% due May 30, 2020 (being the entire original
issue of $10,000,000 in principal amount of such Series N bonds); $12,500,000
principal amount of First Mortgage bonds, Series O, 8.46% due September 30, 2022
(being the entire original issue of $12,500,000 in principal amount of such
Series O bonds); $12,500,000 principal amount of First Mortgage bonds, Series P,
8.09% due September 30, 2022 (being the entire original issue of $12,500,000 in
principal amount of such Series P bonds); $8,000,000 principal amount of First
Mortgage bonds, Series Q, 5.62% due November 30, 2003 (being part of an original
issue of $16,000,000 in principal amount of such Series Q bonds); $15,000,000
principal amount of First Mortgage bonds, Series R, 7.50% due December 15, 2025
(being the entire original issue of $15,000,000 in principal amount of such
Series R bonds); $15,000,000 principal amount of First Mortgage bonds, Series S,
6.82% due April 1, 2018 (being the entire original issue of $15,000,000 in
principal amount of such Series S bonds); and $15,000,000 principal amount of
First Mortgage bonds, Series T, 6.50% due February 1, 2029 (being the entire
original issue of $15,000,000 in principal amount of such Series T bonds); and

     WHEREAS, the Company and Algonquin Gas Transmission Company, a corporation
organized and existing under the laws of the State of Delaware ("Algonquin"),
entered into a Lease with Reservation of Storage Rights dated as of October 1,
1971 (the "Initial Lease"), which Initial Lease provided for, among other
things, the letting by the Company to Algonquin of a parcel of the Company's
land located in the City and County of Providence, Rhode Island, which Initial
Lease was terminated by the Company and Algonquin by that certain Termination of
Lease and Reservation of Storage Rights and Consent Agreement by and among the
Company, Algonquin and Algonquin LNG, Inc., a corporation organized and existing
under the laws of the State of Delaware ("ALNG"), dated as of March 31, 1999;
and

     WHEREAS, the Company and ALNG have entered into a new Lease dated as of
March 31, 1999 (the "Lease"), which Lease provides for, among other things, the
letting by the Company to ALNG of a parcel of the Company's land located in the
City and County of Providence, Rhode Island, that being the same parcel of land
(including the Optioned Premises (as such term is defined in the Initial Lease)
leased by the Company to Algonquin under the Initial Lease (the "Leased
Premises"), and described as follows:

          That portion of Providence's land designated as Parcel A as laid out
          and delineated on that plat
<PAGE>

          entitled "N/F Algonquin LNG, Inc. Lease Areas and Proposed
          Modifications at the ALNG Plant, City of Providence, Rhode Island"
          dated February 18, 1999 prepared by Coler & Colantonio, Inc., 101
          Accord Park Drive, Norwell, Massachusetts 02061, a copy of which is
          annexed hereto, marked Exhibit "A" and made a part hereof. Being a
          portion of land described in a deed from Charles Morris Smith, as
          Trustee and Individually, to Providence recorded in the Office of the
          City Clerk of the City of Providence in Book 470, Pages 224-227. The
          leased premises is also Parcel 316 as shown on Plat 56 of the
          Assessor's Map of the City of Providence dated December 31, 1991 and
          affects portions of Parcel 5 as shown thereon.

          Together with:

               (a)  the right and easement to pass and repass on foot or in
          vehicles (including railroad cars) over any existing ways or currently
          serviceable railroad tracks, if any, designated on Exhibit "A" and to
          refurbish and rehabilitate at its own expense currently unused
          railroad tracks and railroad rights of way through and across that
          portion of Providence's land designated as Parcel B in said Exhibit
          "A";

               (b)  the right and easement to construct and maintain at its own
          expense one or more additional access roads and one or more additional
          railroad spur tracks necessary for the use of the leased premises as
          contemplated herein, said roads and tracks to be at such locations on
          said Parcels C and D and upon such terms as may be mutually agreed
          upon by the parties hereto in writing or which may be incorporated in
          a new Exhibit "A" hereto;

               (c)  the right and easement to construct on said Parcels C and D
          and maintain above or below grade fiber optical, telephone, telegram,
          power, telecommunication and/or electric lines, conduit, poles and/or
          other equipment, water mains and sewer pipes, natural gas transmission
          lines and/or any other facilities necessary for the use of the leased
          premises as contemplated herein, at such locations as shown on Exhibit
          "A" or as may be mutually agreed upon by the parties hereto in
          writing;

               (d)  the right and easement to construct, maintain, operate,
          alter, replace, repair and
<PAGE>

          remove one or more pipelines with valves, tie-overs, and other
          appurtenant facilities necessary for the transmission of liquefied
          natural gas ("LNG"), natural gas and all by-products thereof or any
          liquids, gases, materials or substances which can be transported
          through a pipeline or pipelines, from the dock located on Parcel C, as
          set forth in said Exhibit "A", to the leased premises, at the location
          shown on Exhibit "A" or otherwise to be mutually agreed upon by the
          parties in writing. Providence will grant ALNG said easement by
          separate recordable instrument, upon the written request of ALNG. The
          pipelines across Parcel C shall have a minimum vertical clearance of
          fourteen (14) feet and shall permit the free passage of vehicles
          beneath such pipelines on Parcel C;

               (e)  the right and easement to construct and maintain all
          reasonably necessary facilities on the dock located on Parcel C to
          unload LNG and the other products and substances referred to in the
          foregoing sub-paragraph (d);

               (f)  the right and easement to pass and re-pass on foot or in
          vehicles over all portions of Parcel C, including the dock located
          thereon and to control access for and use of Parcel C in accordance
          with applicable laws, rules, regulations or orders of governmental
          authorities having competent jurisdiction thereover;

               (g)  the easement to use the dock facilities forming a portion of
          Parcel C for the purposes of loading and unloading ships or barges in
          connection with the uses contemplated herein and in connection with
          the construction, operation and maintenance of the facilities
          contemplated herein; and

               (h)  the right and easement to construct, maintain, operate,
          alter, replace, repair and remove one or more pipelines with valves,
          tieovers and other related facilities for the transmission of natural
          gas and all by-products thereof, or any liquids, gases or substances
          which can be transported through a pipeline, from the vaporizers
          constructed or to be constructed by ALNG on the leased premises across
          Parcels C and D, and from Parcel C to Parcel B the locations of such
          pipelines and other facilities as shown on Exhibit "A" or as may be
          mutually agreed upon by the parties hereto in writing. Providence will
          grant ALNG said easement by separate recordable
<PAGE>

          instrument, upon the written request of ALNG. Said rights and
          easements granted by sub-paragraphs (a) through (c) and (e) through
          (g) above, shall be in common with the rights of Providence, its
          tenants and each of their respective licensees, invitees, employees,
          successors and assigns and shall be exercisable by ALNG's licensees,
          invitees and employees in the course of the conduct of activities
          permitted hereunder. The rights and easements granted by subparagraphs
          (d) and (h) shall be exclusive to ALNG; and

     WHEREAS, the Lease further provides for the granting of an option by the
Company to ALNG to include as leased premises under the Lease, the property (the
"St. Lawrence Leased Premises") leased to St. Lawrence Cement, Inc. ("St.
Lawrence") by the Company under that certain Agreement of Lease dated May 23,
1988 between the Company and St. Lawrence recorded in the Providence Land
Records Book 125, Page 312, and Amendment of Lease dated April 3, 1995 between
the Company and St. Lawrence recorded in the Providence Land Records Book 123,
Page 818, in each case modifying a prior Indenture of Lease dated August 1,
1961, recorded in the Providence Land Records Book 1105, Page 239 (the term
"Leased Premises" as used hereinafter being deemed to include the St. Lawrence
Leased Premises to the extent the same is included as leased premises under the
Lease); and

     WHEREAS, the Lease further provides for certain rights and easements in
favor of ALNG with respect to the use of other lands of the Company adjacent to
the Leased Premises; and

     WHEREAS, a memorandum of the Lease, dated as of March 31, 1999, has been
executed by the Company and by ALNG and is to be recorded in the Records of Land
Evidence in the City of Providence immediately prior to the recording of this
Twenty-First Supplemental Indenture; and

     WHEREAS, the Company and the Trustee wish to amend the Indenture to provide
for, among other things, the subordination of the rights of the Trustee under
the Indenture to certain rights of ALNG under said Lease; and

     WHEREAS, the registered holders of more than 80% in aggregate principal
amount of said First Mortgage Bonds outstanding under the Indenture on the date
hereof, by instruments in writing signed by such holders and filed with the
Trustee, have assented to and have authorized the
<PAGE>

Trustee to enter into this Twenty-First Supplemental Indenture and have assented
to and have authorized the modification or alteration of the Indenture as
provided herein;

     NOW, THEREFORE, for good and valuable consideration, the receipt and
sufficiency of which are hereby acknowledged, and in order to permit the
consummation of the Lease:

                                 ARTICLE FIRST

                             Subordination of Lien

     Section 1.01  The Trustee agrees that each and all of the remedies of the
Trustee provided in the Indenture for any default by the Company in the
covenants, stipulations, promises and agreements contained therein, or contained
in any of the bonds issued thereunder, shall be and they hereby are made
subordinate to and subject to each and all of the rights of ALNG under the
Lease, excepting only the right of ALNG to indemnification by the Company under
Section 7 of the Lease and to the covenant of the Company provided in Section
13(3) of the Lease.  Without limiting the foregoing, the Trustee will not for
any reason whatsoever interfere with the use by ALNG of the Leased Premises as
permitted by the terms of the Lease, or the enjoyment by ALNG of the license,
rights and easements granted by the Lease, during the term of the Lease or any
extension thereof, except as otherwise permitted by the terms of this Twenty-
First Supplemental Indenture.

                                ARTICLE SECOND

                             Assignment of Rentals

     Section 2.01  The Company does hereby assign, transfer and set over to the
Trustee, in addition to the mortgage of the Indenture and to any and all other
security or collateral at any time held by it or them, all of the rentals and
other payments that are now due or that hereafter may become due or payable to
the Company or to any subsequent owner of the Leased Premises from all of the
occupants, tenants, lessees, subtenants and sublessees now and from time to time
hereafter occupying said Leased Premises or any portion thereof under and on
account of the Lease and any renewals or extensions thereof, to be held, in
addition to the mortgage and any and all other security at any time held by the
Trustee, as security for the payment and performance of the provisions of the
Company's First
<PAGE>

Mortgage Bonds and of the said mortgage securing the same, and for the payment
and performance of all indebtedness, liabilities and obligations of the Company
to the Trustee, whether now existing or hereafter incurred or acquired, all upon
the terms, conditions, covenants and agreements on the part of the Company and
the Trustee hereinafter set forth.

     Section 2.02  The term "rentals and other payments", as used herein, shall
include all rents, issues and profits from said premises and all other sums now
or hereafter paid or payable to the Company by any occupant, tenant, lessee,
subtenant or sublessee now and from time to time hereafter occupying the Leased
Premises or any portion thereof under or by reason of the Lease, including,
without limitation, proceeds of rent insurance and business interruption
insurance, so-called, awards of damage or other sums paid or payable to the
Company by reason of the taking of all or any portion of the Leased Premises by
condemnation or other similar proceedings, and all sums paid or payable to the
Company in addition to rental for such items as taxes, utilities and water
charges.

     Section 2.03

     (a)  The Company hereby is authorized to execute and perform any and all
amendments, supplements, restatements, extensions, terminations (partial or
otherwise) or other modifications to the Lease (each a "Modification"), so long
as no default by the Company under the Indenture exists before or after giving
effect to such Modification.  Notwithstanding the foregoing, the Company,
without the prior written consent of the Trustee, will not permit any
Modification of the Lease that would amend the provisions of the Lease so as to
change those events which would constitute a default by either party thereunder,
or change either party's remedies upon a default by the other party, in either
case, in a manner that is materially adverse to the Company.  The Company shall
deliver a copy of any Modification of the Lease to the Trustee within 15 days
following the execution thereof, together with a certificate signed and verified
by the President or a Vice President stating that no default by the Company
under the Indenture exists before or after giving effect to such Modification.

     (b)  The Company covenants and agrees that upon and after any event of
default which gives rise to the right of exercise by the Trustee and the
bondholders of their remedies under Article Sixth of the Original Indenture:
<PAGE>

          (i)  the Company will not permit any Modifications of the Lease, nor
     will the Company enter into new leases or agreements affecting the Leased
     Premises or any portion thereof, nor exercise any of the rights, options or
     elections which the Company may now or hereafter be entitled to exercise
     under the provisions of the Lease without the written consent of the
     Trustee first had and received, and does hereby constitute and appoint the
     Trustee as the Company's attorney or attorneys in fact, irrevocable (with
     full power of substitution), for the Company and in the Company's name,
     place and stead, or in the name of the Trustee, to demand, sue for, receive
     and give effectual discharges for all said rentals and other payments, with
     power also to endorse in the name of the Company checks given in payment
     thereof, to institute in the name of the Company or in the name of the
     Trustee such proceedings at law or in equity as the Trustee may deem
     advisable for the purpose of collecting such rentals and other payments or
     for recovery of possession of said Leased Premises or any part thereof in
     the event of any default on the part of any tenant, lessee or occupant
     thereof under the Lease; to give and receive all notices and consents and
     to exercise all rights, options and elections which the Company may be
     entitled to give, receive or exercise under the Lease; in the Trustee's
     discretion to perform on the Company's behalf, if the Trustee deems the
     same advisable (but nothing herein contained and no action taken hereunder
     shall impose upon the Trustee any obligation to do or continue to do so)
     any or all of the obligations of the Company under the provisions of the
     Lease, and to take any action or perform any function in connection with
     the Lease which the Trustee deems necessary or advisable; hereby giving and
     granting unto said attorney or attorneys in fact full power and authority
     to do and perform every act or thing whatsoever requisite, necessary or
     proper to be done under the Lease as fully to all intents and purposes as
     the Company might or could do if personally present, with full power of
     substitution.

          (ii) The rentals and other payments under the Lease so collected by
     the Trustee may, in the sole and uncontrolled discretion of the Trustee,
     after payment of all costs of collection, be received and applied by it
     from time to time, in such order as it may determine, to the payment of
     taxes and assessments assessed upon or with respect to the said Leased
     Premises or any part thereof, or constituting in whole
<PAGE>

     or in part a lien thereof, or to the payment of any other lien or
     encumbrance upon said Leased Premises or any part thereof, whether prior or
     subsequent to the Indenture, or of premiums on such policies insuring the
     Leased Premises, the Company or the Trustee against fire and such other
     hazards or liability as the Trustee may deem advisable, or for the
     reimbursement of any sums expended by the Trustee in performing the
     obligations imposed upon the Company under the provisions of the Lease, or
     upon the Company under the Indenture or any other mortgage covering said
     Leased Premises or any part thereof, or to payments required by the
     provisions of any agreement between the Company and the Trustee covering
     the budgeting or escrow of funds for the payment of taxes, insurance, water
     rates or other charges in connection with the Leased Premises, or the
     Trustee may, in its sole and uncontrolled discretion, but without being
     required so to do, apply any portion of the money so received by it to the
     payment of any expenses (including, without limitation, attorneys' fees)
     incurred or to be incurred by it in the exercise of powers upon it hereby
     conferred or incurred or to be incurred by it or the Company for repairs or
     improvements to or for the maintenance or operation of the Leased Premises
     (the order of such application to be likewise in the sole discretion of the
     Trustee), and the balance of the moneys so received by the Trustee, after
     payment of such and so much of the above-mentioned items as the Trustee
     shall deem proper, if any, may in the like discretion of the Trustee be
     applied to the payment in whole or in part of the interest and principal,
     due or to become due, of any liability, obligation, or indebtedness hereby
     secured, whether matured or unmatured, liquidated or unliquidated, and any
     balance then remaining, likewise in the sole discretion of the Trustee, it
     may continue to hold as collateral or pay over to the Company.

     Section 2.04  The execution and delivery of this Twenty-First Supplemental
Indenture shall not constitute a satisfaction of the obligations, or any part
thereof, which are secured by mortgage granted under the Indenture, except to
the extent of amounts actually received and applied by the Trustee on account of
the same.  Nothing herein contained shall be deemed to obligate the Trustee to
undertake or continue collection of said rentals and other payments hereby
assigned or the management of the Leased Premises at any time it does not choose
to do so, or to take any measure, legal or otherwise, to enforce collection of
<PAGE>

any of said rents or other payments. Notwithstanding the collection of rentals
and other payments hereunder, the Trustee, subject to the provisions of Article
                                                                        -------
FIRST hereof, may institute foreclosure proceedings, sell, realize upon, or
-----
otherwise deal with any security or collateral at any time held by it, and
otherwise exercise any of its rights and powers under the Indenture or
otherwise, in such manner as it may deem advisable, at any time it shall see fit
to do so, and for any cause for which the same might have been instituted or
done had the assignment of rentals herein contained not been made, and no waiver
or acceptance of any breach or default and no waiver of any right of the Trustee
hereunder shall be deemed to constitute a waiver of any other or subsequent
breach or default, or to prevent subsequent exercise of any such right or any
other similar right.

     Section 2.05  The Trustee hereby covenants and agrees as follows:

     (a)  that upon payment in full of all of the indebtedness, liabilities, and
obligations of the Company secured by the assignment of rentals herein contained
whether by application or the rentals and other payments hereby assigned or
otherwise, the Trustee will, upon request, execute and deliver to the Company
all instruments necessary to terminate said assignment and to notify all tenants
and lessees of said Leased Premises of such termination;

     (b)  that notwithstanding anything to the contrary hereinbefore contained,
so long as no default occurs in the payment and performance of all of the
provisions of the Indenture and of the First Mortgage Bonds on the Company's
part to be paid, performed, and observed, the Company shall have the right to
receive, collect, and enjoy the rentals and other payments assigned hereunder,
provided, however, that the foregoing provisions of this subparagraph (b) shall
--------  -------                                        ----------------
immediately upon the occurrence of any such default, become and remain null and
void, inoperative and of no force or effect whatsoever unless and until such
default shall be cured within the grace period, if any, provided for the cure
thereof.

                                 ARTICLE THIRD

                             Additional Provisions

     Section 3.01  The Company, at its own cost and expense, will forthwith,
upon the execution and delivery by
<PAGE>

the parties hereto of this Twenty-First Supplemental Indenture, cause the same
to be recorded, pursuant to law, in the Records of Land Evidence of the City of
Providence, in the State of Rhode Island.

     Section 3.02  This Twenty-First Supplemental Indenture may be executed in
any number of counterparts, each of which shall be deemed an original, and such
counterparts shall constitute but one and the same instrument which shall, for
all purposes, be sufficiently evidenced by any such original counterpart.

     IN WITNESS WHEREOF, the Company has caused its corporate seal to be hereto
affixed and these presents to be executed, acknowledged and delivered in its
name and on its behalf by its President and its Treasurer, and the Trustee has
caused its corporate seal to be hereto affixed and these presents to be
executed, acknowledged and delivered by one of its officers, all hereunto duly
authorized, as of the day and year first above written.

                              THE PROVIDENCE GAS COMPANY


                              By:______________________________
                                  James H. Dodge,
                              President

                              and


                              By:______________________________
                                  Kenneth W. Hogan,
                              Treasurer

(Corporate Seal)

Signed, sealed and delivered in the presence of:


______________________________
Name:_______________________________
As to The Providence Gas Company
<PAGE>

                              STATE STREET BANK & TRUST
                              COMPANY, Trustee


                              By:______________________________
                                      Authorized Officer

(Corporate Seal)

Signed, sealed and delivered in the presence of:


__________________________
Name:_____________________
as to State Street Bank & Trust Company


State of Rhode Island
County of Providence

     In the City of Providence, this ____ day of _________________, 1999,
personally appeared before me James H. Dodge and Kenneth W. Hogan, to me known
and known by me to be the President and Treasurer, respectively, of The
Providence Gas Company, one of the parties that executed the foregoing
instrument, and acknowledged said instrument to be their free act and deed in
their said capacities, and the free act and deed of said corporation, and on
oath stated that they were duly authorized to execute said instrument and that
the seal affixed thereto is the corporate seal of said corporation.

     In witness whereof, I have hereunto set my hand and affixed my official
seal, the day and year first above written.

                              _________________________________
                              Name:  __________________________
                                     Notary Public
                              My Commission Expires: __________
                              ______
SEAL


State of Rhode Island
County of Providence

     In the City of Providence, this ____ day of ___________________, 1999,
personally appeared before me
<PAGE>

_______________________________, to me known and known by me to be an Authorized
Officer of State Street Bank and Trust Company, one of the parties that executed
the foregoing instrument, and acknowledged said instrument to be such person's
free act and deed in such person's said capacities, and the free act and deed of
said corporation, and on oath stated that such person was duly authorized to
execute said instrument and that the seal affixed thereto is the corporate seal
of said corporation.

     In witness whereof, I have hereunto set my hand and affixed my official
seal, the day and year first above written.

                              _____________________________
                              Name:  ______________________
                                     Notary Public
                              My Commission Expires: ______
                              ____

SEAL