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In this task, you will be given a passage to read. A fill in the blank question will be given to you.  Your answer should fit the blank appropriately.

 The second stage of construction in the 17th and 18th centuries , when the tower was under the charge of the Vatican librarian , involved Mgr . Filippo Luigi Gilii , a clergyman of St. Peter 's Basilica . Earlier in 1797 , Pius VI gave approval to placing a Latin inscription Specula Vaticana at the entrance to the upper part of the tower , which was implemented by Cardinal Zelada with plans to enhance the instrumentation system in the tower 's observatory . The original observatory was then set up above the second level of the tower with the agreement of Pope Pius VI . Its instrumentation , apart from many normal devices ( such as meteorological and magnetic equipment , with a seismograph and a small transit and pendulum clock , ) was noted for the Dolland Telescope . The instrumentation facilitated recording of occurrences of eclipse , appearance of comets , Satellites of Jupiter and Mercury ’ s transit . As an addition , under the patronage of Pope Pius X , four rotary observatory domes were also added at strategic locations on the 1 @,@ 300 feet ( 400 m ) long fortification walls , more than a thousand years old . Mgr . Gilii , highly respected as a polyglot with a knowledge of physics , biology , archeology and the Hebrew language , was in charge of the observatory from 1800 to 1821 . He carried out continuous meteorological observations ( twice a day at 6 AM and 2 PM ) conforming to the programme of the Mannheim Meteorological Society . While the observation records for seven years were published , the balance data in a manuscript form was preserved in the Vatican Library . Subsequent to Gilii 's death in 1821 , the observatory on the tower was discontinued and the instruments were moved to the observatory at the Roman College . Established in 1787 , it was considered more suitable for making observations than the Vatican . fill in the blank  :  The second stage of construction in the 17th and __________________ centuries , when the tower was under the charge of the Vatican librarian , involved Mgr . 
Output:
 18th 