[ 9 ] 



The next important article is the foundation, which fhould 

 be of the moft foHd kind : For this reafon a rock, and that 

 of great extent*, or a hard gravel, fliould be made choice of; 

 if neither of thefe can be found in the place where other 

 circumftances require the Obfervatory to be built, it will be 

 necefTary to give all adventitious {lability in our power, by deep 

 arches or piles driven by an engine ; for fuch is the confum- 

 mate execution of modern inftrumcnts, that they immediately 

 betray the imperfedions of a building, whether from unftable 

 foundation, or inartificial fuperflrudure. 



The foil fhould be naturally dry, as will generally be the 

 cafe when the next ftratum is gravel. The advantage of fuch 

 a foil I have frequently remarked at the Royal Obfervatory at 

 Greenwich, during my refidencc near it, where the fecond 

 ftratum, and indeed almoft the firft, being a flinty gravel, I 

 have fometimes feen the air ferene, whilft the country around 

 has been covered with a thick fog; which poflibly may be 

 accounted for from the gravelly ftratum, which fuffers the rain 

 and moift dews to percolate, and leave the furface dry ; or, to 

 indulge another conjedure, perhaps the poliflied furface of the 

 cold flints, with which the foil and furface abound, may tend 

 to decompofe the air, and make it depofit any fuperabundant 

 moiflure. 



The Obfervatory belonging to Trinity College, Dublin, is 

 built on a high ground N. W. of the city, and diftant about 



B four 



* I have heard of an Obfervatory built upon a rock, but one of (hort extent, 

 that betrayed moft Cngular irregularities in the pofition of the inftruments. 



