18 TRANSIT OF MERCUUI'. 



Nay, if the obfervatory fhould be fiirrounded by water, we 

 need be under no apprehenfion on that account. Perhaps, 

 were we to ere<5l a building for aftronomical purpofes only, we 

 ought not to objed to grounds which areoccafionally flooded ; 

 the neighbourhood of a river, a lake, or other generally called 

 damp (ituations. See obfervation 5 and 7. 



It is however poffible, that fogs and hazinefs may increafe 

 to fuch a degree as, at laft, to take away, by their interpofition, 

 all the light which comes from celeftial objefls ; in which cafe, 

 they muft of courfe put an end to obfervation ; but they will 

 neverthelefs be accompanied with diftindl vifion to the very 

 laft. See obfervation 44-, 4-5, and 46. 



We have now only the four laft obfervations to account for. 

 They relate to the change of the focal length of mirrors in 

 folar obfervations, and its attendant derangement of the foci of 

 the different parts of the refleding furface; and, as fimplicity 

 is one of the marks of the truth of a principle, 1 believe we need 

 not have recourfe to any other caufe than the change of tem- 

 perature produced by the adtion of the folar rays thatoccafion 

 heat; which will be quite fufficient to explain all the pheno- 

 mena. Bat, in order to fliow this in its proper light, I fliall 

 relate the following experiments. 



IJl Experiment, 

 Experiments oil J placed a glal"s mirror, of 7 -feet focal length, in the tube 

 focaHength'm belonging to the telefcope ; and, having laid it open at the 

 mirrors by heat, back, I prepared a ftand, on which the iron ufed in my expe- 

 riments on the terreftrial rays that occafion heat (fee Phil. 

 Tranf. for ] 800, Plate XVI. Fig. 1 .) might be placed, (o as to 

 heat the mirror from behind, while I kept a certain obje6l in 

 the field of view of the telefcope. Having meafured the focal 

 length, and alfo examined the figure of the mirror, which was 

 parabolical, the heated iron was applied fo as to be about 2| 

 inches from the back of the glafs mirror. The confequence of 

 this was, that a total confuiion in all the foci took place, fo that 

 the letters on a printed card in view, which before had been 

 extremely diftind, became inftantly illegible. In J 5 feconds, 

 the focus of the mirror was fliortened 2,3 inches; in half a 

 minute, 3,47 inches; and, at the end of the minute, I found 

 it no Jefs than 4,59 inches ftiorter than it had been before the 

 application of the hot iron. 



On 



