December, 1905. 



KNOWLEDGE & SCIENTIFIC NEWS. 



289 



both this and the solstitial marbles of his predecessors 

 to be covered. with a protecting brass,* he at first re- 

 adjusted and then replaced Toscanelli's gfnomon by a 

 new one of the same dimensions in every respect ; his 



avowed object in bestowing- so much care and thought 

 on the apparatus being to bequeath to posterity a 

 means of detecting the slow changes in the obliquity 

 of the ecliptic — the cxiguae edipiuae variaiiones. as he 

 very aptly terms them in the long Latin inscription 

 which is affixed to one of the four massive piers sus- 

 taining the dome. 



But Ximenes' grand design of handing on to future 

 generations an amended edition of this stupendous in- 

 strument of research was soon to be frustrated, for 

 even his careful corrections were not suffered to remain 

 undisturbed. During some repairs to the lantern his 

 gnomon was removed, and deposited for several years 

 ill an adjoining museum ; and, although subsequently 

 restored to its position, it was not brought into proper 

 adjustment until 1893, when Padre Giovann(jzzi,t the 

 present Director of the Ximenian Observatory, again 

 went over the entire work with the greatest precision. 

 Certainly a gnomon subject to fewer vicissitudes might 

 have been obtained had it been possible to carry out 

 the project, once entertained but dismissed as danger- 

 ous, of perforating the great dome itself. 



Graver difficulties, however, than those occasioned 

 by the hand of the restorer conspire tO' render this great 

 solstitial instrument now of small value other than an 



* This had not been raised for fome years, but at my request 

 Signer Pratellesi. the secretary of the Cathedral Office rf Works, 

 most kindly had it removed on the occasion of the summer solstice 

 on June 21st last. 



t To whom, as well as to the Vice-director, Padre ."Vlfani, I am 

 indebted for much historical information. 



historic one. The fact that only for a little over two 

 months at the time of the summer solstice, can the 

 gnomon be used at all, on account of the normally 

 lower altitude of the sun causing its rays to strike the 

 arches of the dome, would not necessarily impair its 

 scientific value; but the beam of light itself, transmitted 

 through the i|-inch aperture of the bronze gnomon, 

 is far too seriously affected by atmospheric perturba- 

 tions, caused by the varj'ing temperature along its ex- 

 tended path from lantern to pavement, to throw more 

 than a very unsteady image on to the meridian below, 

 while that image also, being over four feet in diameter, 

 is of little practical utility from an astronomical point 

 of view; moreover, the expansion due to the sun's 

 heat on the vast building itself would render minute 

 .nccuracy out of the question. 



On the few occasions on which the solstitial observa- 

 tion has been made in recent years, the object has been 

 mainly to detect any slight movement which might 

 have taken place in the fabric of the great Cathedral; 

 and when after the severe earthquake shock of 1895 

 it was found that the trifling errors in position noted 

 may well have been due to the errors in observation 

 inseparable from this mode of investigation, anxiety for 

 the safety of Brunelleschi's wonderful dome gave place 

 to admiration for the work of its architect. To 

 enhance the effect, it is usual, when carrying out the 

 experiment, tO' provide the lantern with a temporary 

 iloorinir, allowing onlv the beam of light from the 



orifice in the gnomon to pass down on to the pavement 

 iDelow, while the transept containing the meridian is 

 also darkened, in order to show the disc of sunlight 

 with greater precision. 



