Account of the Telcfcope at Kiel. 1 17 



utility. The great weight of the fpeculum requires that it 

 mould be always kept in the tube, and this renders neceflary 

 a peculiar apparatus not requifite in fmaller telefcopes. It 

 • is well known that large metalline manes acquire very (low- 

 ly the temperature of the atmofphere. The confequence 

 is, that fpecula are very frequently covered with the evapo- 

 ration from the atmofphere, which condenfes on them. If 

 the compofition of the metal be not good, the fpeculum foon 

 becomes tarnifhed. The caufe of this lies in too large a 

 quantity of copper, which forms a component part of the 

 mafs, and therefore it muft be combined with a fufficient 

 quantity of tin. Edwards's compofition in this refpe6t is 

 much preferable to all other mixtures. By more than forty 

 experiments Profeflbr Schrader difcovered a very good mix- 

 ture ; and fpecula made of it were fo excellent, that they 

 might be expofed to confiderable evaporation without lofing 

 much of their fplendour. As in a clofe apartment no body 

 can be tanaiflied, if a fpeculum be (hut up during hot and 

 moid weather the above inconvenience can be avoided. 

 Profeffbr Schrader has fecured his fpeculum by a cover made 

 of brafs rings, eight inches broad, foldered together. The 

 centre fcrews, as well as the iron circle which keeps the 

 fpeculum in its pofition, are properly fixed in thefe rings, 

 and, by means of two capes with handles, which internally 

 are lined with leather, or, what is better, with the {kins of 

 moles, the fpeculum is kept perfectly free from air. 



The period of the day at which the fpeculum can be un- 

 covered for ufe, depends on the length of time which it re- 

 quires to affume the temperature of the atmofphere. A few 

 experiments will here conduct to the proper regulation. A 

 good rule for guarding againft tarnifhing is, to endeavour 

 as much as pofiible to keep a continual draught of air in 

 the tube. For this purpofe, feveral fmall opening's may be 

 employed in fuch pofitions that the rain may not penetrate 

 through them. A fmall ftage for the purpofe of getting at 

 the fpeculum with more conveniency, is conftru&ed near 

 1 3 the 



