Astronomical Society . 227 



The means which I have employed to ascertain the effect of tem- 

 perature upon the measures by the heliometer, consist in observing 

 such C'f the stars of the Pleiades as are visible in the coldest winter, 

 by night, and in the warmest summer, by day. Soon after the in- 

 strument was set up (in November and December 1829) I made a 

 series of observations of this kind, and repeated them in the summer 

 of 1830. From these I found the value of one revolution of the 

 screw in the temperature / of Fahrenheit, 



= J2"-91788 — (/— 49-2)0"-0004493 {Astro. Nach. No. 189, p. 418). 



Further obsei-vations, however, have reduced the value of the last 

 term of the formula to 0"-0003912 : this latter value is that which 

 I have employed in the reduction of the obsen'ations of 61 Cygni. 

 If this correction of the measures had been altogether neglected, the 

 result, which the star a affords, would have been in error about 

 0''-06 ; but, in the case of the star h, the effect would be altogether 

 inappreciable, since the maximum of the influence in question takes 

 pliice at that time of the year in which the parallax disappears. I 

 owe this explanation to you, since you have inquired expressly as 

 to this point; and, moreover, it could not be indifferent to me that 

 an astronomer to whose opinion I attach so much importance, should 

 not onlj' be partially, but also thoroughly, satisfied as to the paral- 

 lax of 61 Cygni. 



After nine years' service, I resolved to take the heliometer to 

 pieces, in order to examine anew all the parts of the mechanism of 

 this very ponderous instrument ; and to provide, in time, against any 

 damage it may have sustained. The whole, however, is so solidly 

 and durably constructed, that it has been found to need scarcely any 

 repair. I have taken the opportunity of making some alterations 

 for the greater convenience of the observer. The instrument, on 

 account of these circumstances, has been nearly three weeks out of 

 use ; it is now, however, again in a fit state for observation. 



I am particularly anxious to obtain j'our physical observations of 

 the comet. Struve has lately communicated to me his own, which 

 differ considerably from mine, as they show the tail defined, whereas 



it appeared to me undefined ; y\_y Struve, y^^Bessel. In other 



respects these observations are similar to mine, except that they go 

 more into detail. It appears that I am the only one who has had 

 the good fortune to be able to follow the comet during an entire 

 night, in which the motion of the tail fell in its own direction. Ac- 

 cording to the letter whicli I had the pleasure to receive from you, 

 the comet seems latterly to have lost its tail altogether ; at least you 

 mention oidy the complete definition of the disc, which also I con- 

 sider a very important observation. 



At the approaching disappearance of Saturn's ring, sufficiently 



tract relates, will be found in our number for January, present volume, 

 p. 08. — Edit. 



Q2 



