1
                                                                    Exhibit 3.21

                                     BY-LAWS
                                       OF
                                   NDSI, INC.

              -----------------------------------------------------


                                   ARTICLE I.
                                     OFFICES

         The office of the Corporation shall be located in the City and
State designated in the Articles of Incorporation. The Corporation may also
maintain offices at such other places within or without the United States as the
Board of Directors may, from time to time, determine.

                                   ARTICLE II.
                             MEETING OF SHAREHOLDERS

                  Section 1. Annual Meetings. The annual meeting of the
shareholders of the Corporation shall be held within five months after the close
of the fiscal year of the Corporation, for the purpose of electing directors,
and transacting such other business as may properly come before the meeting.

                  Section 2. Special Meetings. Special meetings of the
shareholders may be called at any time by the Board of Directors or by the
President, and shall be called by the President or the Secretary at the written
request of the holders of ten per cent (10%) of the shares then outstanding and
entitled to vote thereat, or as otherwise required under the provisions of the
Business Corporation Act.

                  Section 3. Place of Meetings. All meetings of shareholders
shall be held at the principal office of the Corporation, or at such other
places as shall be designated in the notices or waivers of notice of such
meetings.


                                   By-Laws - 1
   2
                  Section 4. Notice of Meetings. (a) Except as otherwise
provided by Statute, written notice of each meeting of shareholders, whether
annual or special, stating the time when and place where it is to be held, shall
be served either personally or by mail, not less than ten or more than fifty
days before the meeting, upon each shareholder of record entitled to vote at
such meeting, and to any other shareholder to whom the giving of notice may be
required by law. Notice of a special meeting shall also state the purpose or
purposes for which the meeting is called, and shall indicate that it is being
issued by, or at the direction of, the person or persons calling the meeting.
If, at any meeting, action is proposed to be taken that would, if taken, entitle
shareholders to receive payment for their shares pursuant to Statute, the notice
of such meeting shall include a statement of that purpose and to that effect. If
mailed, such notice shall be directed to each such shareholder at his address,
as it appears on the records of the shareholders of the Corporation, unless he
shall have previously filed with the Secretary of the Corporation a written
request that notices intended for him be mailed to some other address, in which
case, it shall be mailed to the address designated in such request.

                  (b) Notice of any meeting need not be given to any person who
may become a shareholder of record after the mailing of such notice and prior to
the meeting, or to any shareholder who attends such meeting, in person or by
proxy, or to any shareholder who, in person or by proxy, submits a signed waiver
of notice either before or after such meeting. Notice of any adjourned meeting
of shareholders need not be given, unless otherwise required by statute.

                  Section 5. Quorum. (a) Except as otherwise provided herein, or
by statute, or in the Certificate of Incorporation (such Certificate and any
amendments thereof being hereinafter collectively referred to as the
"Certificate of Incorporation"), at all meetings of shareholders of the
Corporation, the presence at the commencement of such meetings in person or by
proxy of shareholders holding of record a majority of the total number of shares
of the Corporation then issued and outstanding and entitled to vote, shall be
necessary and sufficient to constitute a quorum for the transaction of any
business. The withdrawal of any shareholder after the commencement of a meeting
shall have no effect on the existence of a quorum, after a quorum has been
established at such meeting.

                                   By-Laws - 2
   3
                  (b) Despite the absence of a quorum at any annual or special
meeting of shareholders, the shareholders, by a majority of the votes cast by
the holders of shares entitled to vote thereon, may adjourn the meeting. At any
such adjourned meeting at which a quorum is present, any business may be
transacted at the meeting as originally called if a quorum had been present.

                  Section 6. Voting. (a) Except as otherwise provided by statute
or by the Certificate of Incorporation, any corporate action, other than the
election of directors, to be taken by vote of the shareholders, shall be
authorized by a majority of votes cast at a meeting of shareholders by the
holders of shares entitled to vote thereon.

                  (b) Except as otherwise provided by statute or by the
Certificate of Incorporation, at each meeting of shareholders, each holder of
record of stock of the Corporation entitled to vote thereat, shall be entitled
to one vote for each share of stock registered in his name on the books of the
Corporation.

                  (c) Each shareholder entitled to vote or to express consent or
dissent without a meeting, may do so by proxy; provided, however, that the
instrument authorizing such proxy to act shall have been executed in writing by
the shareholder himself, or by his attorney-in-fact thereunto duly authorized in
writing. No proxy shall be valid after the expiration of eleven months from the
date of its execution, unless the person executing it shall have specified
therein the length of time it is to continue in force. Such instrument shall be
exhibited to the Secretary at the meeting and shall be filed with the records of
the Corporation.




                                   By-Laws - 3
   4
                  Section 7. Manner of Acting. (a) At all meetings of the Board
of Directors, each director present shall have one vote, irrespective of the
number of shares of stock, if any, which he may hold.

                   (b) Except as otherwise provided by statute, by the
Certificate of Incorporation, or by these By-Laws, the action of a majority of
the directors present at any meeting at which a quorum is present shall be the
act of the Board of Directors. Any action authorized, in writing, by all of the
directors entitled to vote thereon and filed with the minutes of the corporation
shall be the act of the Board of Directors with the same force and effect as if
the same had been passed by unanimous vote at a duly called meeting of the
Board.

                  Section 8. Vacancies. Any vacancy in the Board of Directors
occurring by reason of an increase in the number of directors, or by reason of
the death, resignation, disqualification, removal (unless a vacancy created by
the removal of a director by the shareholders shall be filled by the
shareholders at the meeting at which the removal was effected) or inability to
act of any director, or otherwise, shall be filled for the unexpired portion of
the term by a majority vote of the remaining directors, though less than a
quorum, at any regular meeting or special meeting of the Board of Directors
called for that purpose.

                  Section 9. Resignation. Any director may resign at any time by
giving written notice to the Board of Directors, the President or the Secretary
of the Corporation. Unless otherwise specified in such written notice, such
resignation shall take effect upon receipt thereof by the Board of Directors or
such

                                   By-Laws - 4
   5
officer, and the acceptance of such resignation shall not be necessary to make
it effective.

                  Section 10. Removal. Any director may be removed with or
without cause at any time by the affirmative vote of shareholders holding of
record in the aggregate at least a majority of the outstanding shares of the
Corporation at a special meeting of the shareholders called for that purpose,
and may be removed for cause by action of the Board.

                  Section 11. Salary. No stated salary shall be paid to
directors, as such, for their services, but by resolution of the Board of
Directors a fixed sum and expenses of attendance, if any, may be allowed for
attendance at each regular or special meeting of the Board; provided, however,
that nothing herein contained shall be construed to preclude any director from
serving the Corporation in any other capacity and receiving compensation
therefor.

                  Section 12. Contracts. (a) No contract or other transaction
between this Corporation and any other Corporation shall be impaired, affected
or invalidated, nor shall any director be liable in any way by reason of the
fact that any one or more of the directors of this Corporation is or are
interested in, or is a director or officer, or are directors or officers of such
other Corporation, provided that such facts are disclosed or made known to the
Board of Directors.

                  (b) Any director, personally and individually, may be a party
to or may be interested in any contract or transaction of this Corporation, and
no director shall be liable in any way by reason of such interest, provided that
the fact of such interest be disclosed or made known to the Board of Directors,
and provided that the Board of Directors shall authorize, approve or ratify such
contract or transaction by the vote (not counting the vote of any such director)
of a majority of a quorum, notwithstanding the presence of any such director at
the meeting at which such action is taken. Such director or directors may be
counted in determining the presence of a quorum at such meeting. This Section
shall not be construed to impair or invalidate or in any way affect any contract
or other transaction which would otherwise be valid under the law (common,
statutory or otherwise) applicable thereto.

                                   By-Laws - 5

   6
                  Section 13. Committees. The Board of Directors, by resolution
adopted by a majority of the entire Board, may from time to time designate from
among its members an executive committee and such other committees, and
alternate members thereof, as they may deem desirable, each consisting of three
or more members, with such powers and authority (to the extent permitted by law)
as may be provided in such resolution. Each such committee shall serve at the
pleasure of the Board.

                                   ARTICLE IV.

                                    OFFICERS

                  Section 1. Number, Qualifications, Election and Term of
Office. (a) The officers of the Corporation shall consist of a President, a
Secretary, a Treasurer, and such other officers, including a Chairman of the
Board of Directors, and one or more Vice Presidents as the Board of Directors
may from time to time deem advisable. Any officer other than the Chairman of the
Board of Directors may be, but is not required to be, a director of the
Corporation. Any two or more offices may be held by the same person.

                  (b) The officers of the Corporation shall be elected by the
Board of Directors at the regular annual meeting of the Board following the
annual meeting of shareholders.

                  (c) Each officer shall hold office until the annual meeting of
the Board of Directors next succeeding his election, and until his successor
shall have been elected and qualified, or until his death, resignation or
removal.

                  Section 2. Resignation. Any officer may resign at any time by
giving written notice of such resignation to the Board of Directors, or to the
President or the Secretary of the Corporation. Unless otherwise specified in
such written notice, such resignation shall take effect upon receipt thereof by
the Board of Directors or by such officer, and the acceptance of such
resignation shall not be necessary to make it effective.

                                   By-Laws - 6
   7
                  Section 3. Removal. Any officer may be removed, either with or
without cause, and a successor elected by a majority vote of the Board of
Directors at any time.

                  Section 4. Vacancies. A vacancy in any office by reason of
death, resignation, inability to act, disqualification, or any other cause, may
at any time be filled for the unexpired portion of the term by a majority vote
of the Board of Directors.

                  Section 5. Duties of Officers. Officers of the Corporation
shall, unless otherwise provided by the Board of Directors, each have such
powers and duties as generally pertain to their respective offices as well as
such powers and duties as may be set forth in these by-laws, or may from time to
time be specifically conferred or imposed by the Board of Directors. The
President shall be the chief executive officer of the Corporation.

                  Section 6. Sureties and Bonds. In case the Board of Directors
shall so require, any officer, employee or agent of the Corporation shall
execute to the Corporation a bond in such sum, and with such surety or sureties
as the Board of Directors may direct, conditioned upon the faithful performance
of his duties to the Corporation, including responsibility for negligence and
for the accounting for all property, funds or securities of the Corporation
which may come into his hands.

                  Section 7. Shares of Other Corporations. Whenever the
Corporation is the holder of shares of any other Corporation, any right or power
of the Corporation as such shareholder (including the attendance, acting and
voting at shareholders' meetings and execution of waivers, consents, proxies or
other instruments) may be exercised on behalf of the Corporation by the
President, any Vice President, or such other person as the Board of Directors
may authorize.


                                   By-Laws - 7
   8
                                   ARTICLE V.

                                 SHARES OF STOCK

                  Section 1. Certificate of Stock. (a) The certificates
representing shares of the Corporation shall be in such form as shall be adopted
by the Board of Directors, and shall be numbered and registered in the order
issued. They shall bear the holder's name and the number of shares, and shall be
signed by (i) the Chairman of the Board or the President or a Vice President,
and (ii) the Secretary or Treasurer, or any Assistant Secretary or Assistant
Treasurer, and shall bear the corporate seal.

                  (b) No certificate representing shares shall be issued until
the full amount of consideration therefor has been paid, except as otherwise
permitted by law.

                  (c) To the extent permitted by law, the Board of Directors may
authorize the issuance of certificates for fractions of a share which shall
entitle the holder to exercise voting rights, receive dividends and participate
in liquidating distributions, in proportion to the fractional holdings; or it
may authorize the payment in cash of the fair value of fractions of a share as
of the time when those entitled to receive such fractions are determined; or it
may authorize the issuance, subject to such conditions as may be permitted by
law, of scrip in registered or bearer form over the signature of an officer or
agent of the Corporation, exchangeable as therein provided for full shares, but
such scrip shall not entitle the holder to any rights of a shareholder, except
as therein provided.

                  Section 2. Lost or Destroyed Certificates. The holder of any
certificate representing shares of the Corporation shall immediately notify the
Corporation of any loss or destruction of the certificate representing the same.
The Corporation may issue a new certificate in the place of any certificate
theretofore issued by it, alleged to have been lost or destroyed. On production
of such evidence of loss or destruction as the Board of Directors in its
discretion may require, the Board of Directors may, in its discretion, require
the owner of the lost or destroyed certificate, or his legal representatives, to
give the Corporation a bond in such sum as the Board may direct, and with such
surety or sureties as may

                                   By-Laws - 8
   9
be satisfactory to the Board, to indemnify the Corporation against any claims,
loss, liability or damage it may suffer on account of the issuance of the new
certificate. A new certificate may be issued without requiring any such evidence
or bond when, in the judgment of the Board of Directors, it is proper so to do.

                  Section 3. Transfers of Shares. (a) Transfers of shares of the
Corporation shall be made on the share records of the Corporation only by the
holder of record thereof, in person or by his duly authorized attorney, upon
surrender for cancellation of the certificate or certificates representing such
shares, with an assignment or power of transfer endorsed thereon or delivered
therewith, duly executed, with such proof of the authenticity of the signature
and of authority to transfer and of payment of transfer taxes as the Corporation
or its agents may require.

                  (b) The Corporation shall be entitled to treat the holder of
record of any share or shares as the absolute owner thereof for all purposes
and, accordingly, shall not be bound to recognize any legal, equitable or other
claim to, or interest in, such share or shares on the part of any other person,
whether or not it shall have express or other notice thereof, except as
otherwise expressly provided by law.

                  Section 4. Record Date. In lieu of closing the share records
of the Corporation, the Board of Directors may fix, in advance, a date not
exceeding fifty days, nor less than ten days, as the record date for the
determination of shareholders entitled to receive notice of, or to vote at, any
meeting of shareholders, or to consent to any proposal without a meeting, or for
the purpose of determining shareholders entitled to receive payment of any
dividends, or allotment of any rights, or for the purpose of any other action.
If no record date is fixed, the record date for the determination of
shareholders entitled to notice of or to vote at a meeting of shareholders shall
be at the close of business on the day next preceding the day on which notice is
given, or, if no notice is given, the day on which the meeting is held; the
record date for determining shareholders for any other purpose shall be at the
close of business on the day on which the resolution of the directors relating
thereto is adopted. When a determination of shareholders of record entitled to
notice of or to vote at any meeting of shareholders has been

                                  By-Laws - 9
   10
made as provided for herein, such determination shall apply to any adjournment
thereof, unless the directors fix a new record date for the adjourned meeting.


                                   ARTICLE VI.

                                    DIVIDENDS

         Subject to applicable law, dividends may be declared and paid out of
any funds available therefor, as often, in such amounts, and at such time or
times as the Board of Directors may determine.

                                  ARTICLE VII.

                                   FISCAL YEAR

                  The fiscal year of the Corporation shall be fixed by the Board
of Directors from time to time, subject to applicable law.

                                  ARTICLE VIII.

                                 CORPORATE SEAL

                  The corporate seal, if any, shall be in such form as shall be
approved from time to time by the Board of Directors.

                                   ARTICLE IX.

                                   AMENDMENTS

                  Section 1. By Shareholders. All by-laws of the Corporation
shall be subject to alteration or repeal, and new by-laws may be made, by the
affirmative vote of shareholders holding of record in the aggregate at least a
majority of the outstanding shares entitled to vote in the election of directors
at any annual or special meeting of shareholders, provided that the notice or
waiver

                                  By-Laws - 10
   11
of notice of such meeting shall have summarized or set forth in full therein,
the proposed amendment.

                  Section 2. By Directors. The Board of Directors shall have
power to make, adopt alter, amend and repeal, from time to time, by-laws of the
Corporation; provided, however, that the shareholders entitled to vote with
respect thereto as in this Article IX above-provided may alter, amend or repeal
by-laws made by the Board of Directors, except that the Board of Directors shall
have no power to change the quorum for meetings of shareholders or of the Board
of Directors, or to change any provisions of the by-laws with respect to the
removal of directors or the filling of vacancies in the Board resulting from the
removal by the shareholders. If any by-law regulating an impending election of
directors is adopted, amended or repealed by the Board of Directors, there shall
be set forth in the notice of the next meeting of shareholders for the election
of directors, the by-law so adopted, amended or repealed, together with a
concise statement of the changes made.

                                   ARTICLE X.

                                    INDEMNITY

                  (a) Any person made a party to any action, suit or proceeding,
by reason of the fact that he, his testator or intestate representative is or
was a director, officer or employee of the Corporation, or of any Corporation in
which he served as such at the request of the Corporation, shall be indemnified
by the Corporation against the reasonable expenses, including attorney's fees,
actually and necessarily incurred by him in connection with the defense of such
action, suit or proceedings, or in connection with any appeal therein, except in
relation to matters as to which it shall be adjudged in such action, suit or
proceeding, or in connection with any appeal therein that such officer, director
or employee is liable for negligence or misconduct in the performance of his
duties.

                  (b) The foregoing right of indemnification shall not be deemed
exclusive of any other rights to which any officer or director or employee may
be entitled apart from the provisions of this section.

                                  By-Laws - 11
   12
                  (c) The amount of indemnity to which any officer or any
director may be entitled shall be fixed by the Board of Directors, except that
in any case where there is no disinterested majority of the Board available, the
amount shall be fixed by arbitration pursuant to the then existing rules of the
American Arbitration Association.


                  The undersigned Incorporator certifies that he has adopted the
foregoing by-laws as the first by-laws of the Corporation.

Dated: as of November 5, 1983

                                                       /s/ Samuel R. Newborn
                                                       _________________________
                                                              Incorporator
                                                           Samuel R. Newborn


                                  By-Laws - 12