THIRD AMENDMENT OF SUBORDINATION AGREEMENT

     THIS THIRD AMENDMENT OF SUBORDINATION AGREEMENT (this "Amendment"), dated
as of March 11, 1999, is between SLOUGH ESTATES USA INC. ("Slough Estates"),
f/k/a SLOUGH PARKS INCORPORATED ("Slough Parks"), and U.S. BANK NATIONAL
ASSOCIATION ("USB"), f/k/a COLORADO NATIONAL BANK ("CNB").

                                       RECITALS

     A.   Slough Parks and CNB executed and delivered a Subordination Agreement
dated as of December 20, 1996 (the "Subordination Agreement"), pursuant to
which, among other things, Slough Parks subordinated certain obligations owed to
Slough Parks by Tipperary Corporation, a Texas corporation ("Borrower"), to any
and all obligations owed to CNB by Borrower under or in connection with a
Revolving Credit and Term Loan Agreement dated as of March 30, 1992, as amended
(the "Loan Agreement"), among Borrower, et al. and CNB.  The Subordination
Agreement was afterwards amended by an Amendment to Subordination Agreement and
Consent of Subordinating Party, dated as of February 13, 1998, between Slough
Parks and USB, and a Second Amendment of Subordination Agreement, dated as of
December 22, 1998, between Slough Estates and USB (the "Second Amendment").

     B.   USB is the successor to CNB.  Slough Estates is the same entity as
Slough Parks.

     C.   Borrower executed a Promissory Note, dated December 22, 1998, in the
principal amount of $5.5 million (the "Replacement Subordinated Note"), and
Borrower is executing a Promissory Note, dated March 11, 1998, in the principal
amount of $6.5 million (the "Second Replacement Note"), in consideration of the
cancellation of the Replacement Subordinated Note and receipt of $1.0 million
from Slough Estates.  The Second Replacement Note shall constitute all or a part
of the "Subordinated Obligations" as defined in the Subordination Agreement.

     D.   In connection with the Second Replacement Note, the Replacement
Subordinated Note, as defined in the Second Amendment, is being repaid.  In an
Amendment to Security Agreement, of even date herewith, the Security Agreement,
dated December 22, 1998, is being amended to provide that it will secure
repayment of the Second Replacement Note instead of the Replacement Subordinated
Note.  The foregoing Security Agreement and Amendment to Security Agreement
together shall hereafter be referred to as the "Security Agreement."

     E.   The execution and delivery of this Amendment by Slough Estates is
required in order for the Replacement Subordinated Note and Security Agreement
to be in conformity with the Loan Agreement.

                                      AMENDMENT

     NOW THEREFORE, in consideration of $10.00 and other good and valuable
consideration, the receipt and sufficiency of which are hereby acknowledged:

     1.   As of, and after the execution of this Amendment, all references in
the Subordination Agreement (i) to the Subordinated Note shall be deemed to be
to the Second Replacement Note, as herein defined, and (ii) to the Pledged
Interest shall continue, as set forth above and in the Second Amendment, to be
deemed to be to the Replacement Pledged Interest, as defined in the Second
Amendment, provided that the exclusion from the Replacement Pledged Interest set
forth in clause (1) of Recital C of the Second Amendment shall also be deemed to
cover the properties added to the



mortgage described in said clause (1) pursuant to an amendment to the mortgage
dated as of February 16, 1999.

     2.   This Amendment may be executed in any number of counterparts, each of
which shall be an original and no one of which need be signed by all of the
parties, but all of which together shall constitute one and the same instrument.

     3.   Slough Estates hereby ratifies the Subordination Agreement and
confirms that it remains valid, enforceable and in full force and effect, as
amended.

SLOUGH ESTATES USA INC.                 U.S. BANK NATIONAL ASSOCIATION
   f/k/a SLOUGH PARKS INCORPORATED         f/k/a COLORADO NATIONAL BANK


By:  /s/ R. W. Rohner                   By:  /s/ Charles S. Searle
   ---------------------------------       ------------------------------------
   Randall W. Rohner, Vice President       Charles S. Searle, Senior Vice 
   and chief Financial Officer             President

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