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 revival of the observatory on the Gregorian Tower was initiated by the Barnabite Francesco Denza with the approval of Pope Leo XIII . High quality instruments were procured , partly with generous donations from Hicks of London , and the automatic recording instruments were procured from Richard in Paris . A four @-@ inch equatorial , a three @-@ inch transit instrument , and four pendulum clocks with two chronometers , were also procured from the observatory at Modena . In 1888 , the gift of a 16 inch long telescope to Pope Leo XIII , became a part of the observatory . Father Denza joined the observatory in 1889 after it was upgraded with more modern instruments . The same year , a second tower was built some 400 metres ( 1 @,@ 300 ft ) away from the main Gregorian Tower , overlooking the Vatican Gardens behind St. Peter 's Basilica on the south @-@ west border . It was built to a diameter of 17 metres ( 56 ft ) with a lower wall thickness of 4 @.@ 5 metres ( 15 ft ) , which could bear the load of a 13 inch photographic refractor , newly procured from Paris . Augustinian Father Rodriguez was the expert meteorologist who held the post of director from 1898 to 1905 . In 1891 , Pope Leo XIII , promulgating the motu proprio Ut mysticam , designated the second tower as the seat of the newly established Vatican Observatory , a decision which required altering the roof to provide a flat terrace for astronomical observations . fill in the blank  : The revival of the observatory on the __________________ Tower was initiated by the Barnabite Francesco Denza with the approval of Pope Leo XIII . 
 Gregorian 