Est-il clair que je n'insiste nullement et que je n'in- 

 sistais jamais sur la parabole? 



Bond a parfaitement compris ma Noie du № 1291 

 des Astr. Nachr. Voici ce qu'on trouve dans le №1339 

 de ce journal. Après avoir examiné et comparé entre 

 eux 123 dessins de la tète de la comète Donati faits à 

 15 observatoires, du 24 août au 24 octobre, Bond en 

 déduit pour le 2 octobre la courbe normale du contour 

 de la tête. Cette courbe, ayant le même sommet avec 

 la parabole, est moins divergente que celle-ci et coïncide 

 presque avec une chaînette, et Bond dit: «The precise 

 agreement of the outline with catenary in the present 

 instance may be partly accidental, but many other co- 

 mets present a similar caracter. 



In № 1291 of the Astr. Nachr., Prof. Bredichin has 

 shown that by neglecting the radius of the sphere of at-, 

 traction of the nucleus and assuming, that g is the same 

 for all values of G, Bessels formulae give for the curve 

 of the exterior contour of the head of a comet, a para- 

 bola with the nucleus at the focus. 



In the American Journal of science XXVII, p. 87, and 

 XXIX, p. 384, Prof. Norton has derived the parabola for 

 the limiting figure of the head both for the case supposed 

 by Bredichin, and also when there is superadded a small 

 force of repulsion directed from the nucleus. 



To effect a better agreement with observation, we 

 may suppose that the emissions from the nucleus are li- 

 mited in direction to a small range on either side of 

 the line from the nucleus to the sun, so that sin G 

 shall not have large values. 



This does not however accord with the appearance of 

 the enveloppes in their earlier stages, or we may ass- 



