Observations of the Transit of Mercury. 189 



sighted, and the air at his station was undoubtedly very 

 clear and steady. Is it not possible that Mr. Landreth 

 may have seen the planet projected against the solar 

 "sierra," or some part of the solar envelope, not ordinarily 

 visible in telescopic observation ? Of the reality of the 

 phenomenon as It appeared to him I entertain no doubt. 

 Professor Doolittle remarks that his Contact II appeared 

 to be late at the time of observation. Mr. Pritchett's 

 observations may be afi'ected by an error in his assumed 

 value of the longitude of his observatory, which depends, 

 at present, on observed occultations of stars by the moon. 

 Moreover, I have used for internal contacts another phase 

 than that indicated by him, which latter, from all I can 

 gather, is that of " geometrical " contact. 



During the transit and through intervals in the clouds, 

 I obtained thirteen tolerably good micrometrical measures 

 of the diameter of Mercury. Uncorrected for irradiation 

 and possible expansion of the micrometer screw, the result 

 is: 11''.30 zb".14. The observations were obtained with 

 great difficulty, but are believed to be as reliable as could 

 be expected from the smallness of the number of measures. 

 Taking this diameter as the " Transit" diameter we have 

 as the true interval between Contacts I and II, as well as 

 between III and lY : 



187.-5 X = 176."6. 



If now, we suppose that for the average of observers 

 external contact is retarded at ingress and accelerated at 

 egress six seconds, we have as the true interval 170.'6. This 

 seems to correspond well with the observations. Giving 

 equal weight to all observations of a given contact, which 

 are not included in brackets, we have : 



