[ '5 ] 



Weft. Thefe were chofen as they lay befide each other in the 

 quarry ; and though each be a heterogeneous mafs, yet their 

 relative parts at given altitudes are perfedly fimilar ; and this 

 appears abfolutely neceffary to prevent any efFetls of diffimilar 

 expanfion or contradion from heat, cold, moifture, &c. at 

 given heights. If Portland ftone were perfedly homogeneous this 

 would be an unneceffary caution, but the flighteft attention will 

 prove that it is not fo ; and thefe pillars particularly {hew it. 

 Further, experiment proves to us that the temperature of the 

 pillars is different at different altitudes ; if now the two pillars 

 at a given altitude have a given temperature, and happen to be 

 diffimilar at that altitude, it is natural to expe£t that their ex- 

 panfions will be different, and the adjuftment of the inftrument 

 fupported by them deftroyed. 



Each of the fupporting pillars confifting of one folid piece, 

 all effedls of mortar and cement are avoided, and what is of 

 more importance, all iron cramps are unneceffary. 



The temperature of the pillars at different heights is fhewn 

 by thermometers, the tubes of which are bent at right angles, 

 and their bulbs are inferted into the ftone, and furrounded with 

 duft of the fame ftone. 



Near the weftern end of the crofs arife four pillars, marked 

 M, M, M, M, for the fupport of the frame of the vertical meridian 

 circle *. Beneath, from North to South, is laid another block of 



Portland 



* Inftead of quadrants attached to a ftone pillar, it is propofed here to have 

 an entire circle of ten feet diameter, , fupported on a vertical axis : This alteration 



feems 



