[ 17 ] 



of duft, which might in time form a communication between the 

 mafonry and the floor. 



It has not been ufual in Obfervatories to place the tranfit 

 inftrument and thofe adopted for meafuring altitudes in the fame 

 room, and yet fome advantages arife from it. In the firft 

 place, one clock fuffices, which is a circumftance of fome ceco- 

 nomy, where the price is eighty guineas. But the follow- 

 ing is a flngular convenience : it not unfrequently happens 

 that one perfon is obliged to take both the meridian tranfit and 

 the altitude of a celeftial object, in which cafe thefe inftruments 

 being placed near each other afford a ftriking advantage. The 

 obferver may take the pafTage of the fun's limb (for inftancc) over 

 the two "firft wj^es of the tranfit inftrument, omit the middle 

 wire, haften to the circle, take the altitude of one limb, and 

 write it down, then take the altitude of the other, and leave it 

 to be read off" at leifure, return to the tranfit inftrument, and 

 take the pafiTage over the two laft wires, whence the right afcen- 

 fion can be determined with as much accuracy as if the pafi'age 

 over the meridian wire itfelf had been obfcrvcd. 



So far we have confulted ftabihty and convenient difpofition ; 

 the provifion for equability of temperature is next to be con- 

 fidered. 



It is manifeft that obfervations with fuch large inftruments 

 cannot be made abfolutely in the open air ; all we have left 

 therefore is to admit as free a pafiTage to the external air as is 

 confiftent with the fafety of the inftruments and the obferver. 



C • For 



