414 Rev, Mr. Emmett's Observations [Dec* 



After the most careful observations continued during the 

 whole time that Venus was visible^ I have not been able to 

 arrive at any certain conclusion respecting her period beyond 

 this, that a change in the place of the spots could be perceived 

 ilk the space of two or three hours, when the air was in a serene 

 state : this agrees with the period assigned by Cassini. The 

 direction of their motion, deduced from these observations, 

 agrees also with the position of the planet's axis, as determined 

 by those astronomers. I noted down all the observations at the 

 time they were made ; and when the series was completed, cal- 

 culated the position of the axis, as seen from the earth on the 

 20th April. The sun's place was 0° 5' 18''; Venus's helioc. 

 Longitude £i: IP 13'; her N pole is directed towards ^ 20°, 

 and elevated about 15° above the plane of the ecliptic ; hence 

 the N pole was 16° 49' from the W limb, or ^-if- of the planet's 

 radius ; the illuminated part of the disc was 55° 49', or -^^^ of 

 her radius ; so that the distance of the N pole from the W limb 

 was about one-tenth of the illu- 

 minated part. In the figure, the 

 position of the pole is shown, as /^ 



the planet appeared in the tele- 

 scope, inverted and reversed ; the 



circular arcs show the apparent ,. ^^ 



paths of the spots. ^ 



It is evident that the position 

 of the planet was very unfavourable to these observations. 



It is to be hoped that astronomers will in future make parti- 

 cular observations upon this planet. It is certainly very remark- 

 able that for a century no spots have-ever been seen, except once 

 by Short ; this may be partly ascribed to the instruments in use, 

 for I see them best with the refractors. Several persons have 

 seen them through my telescopes, and their drawings and descrip- 

 tions always coincided with mine ; therefore there could be no 

 fallacy. I am fully persuaded that if the old aerial telescope 

 were more generally appUed, not only the spots of Venus, but 

 other objects, might be better seen than with other instruments. 

 I saw them better defined than with the reflector, the powers 

 being equal ; the same spots could not be seen with a good 

 achromatic of 2^ inches aperture, and very imperfectly by one 

 of 3 inches, by Dollond, with higher powers than I used in the 

 others. I examined the planet in hopes of seeing some appear- 

 ance of mountains, which some observers speak of: I employed 

 powers from 70 to 800 : with the latter I see the double ring and 

 all the belts of Saturn, and close double stars in great perfection ; 

 but in no instance could I perceive any trace of them, although 

 I paid particular attention to the concave edge, sometimes with- 

 out a screen glass ; at others employing smoked glasses of every 

 variety of shade. J. 6. Emmett. 



