W. A. Norton on Comets. 99 
In the instances of the Comets of 1799 and 1807 the date of 
the measurements is not known, and aceordingly the distance 
been made from the observed diameters. The results obtained 
by taking the diameter of nucleus at 400 miles are given at the 
Name. Oeiapdiot eh Sun. Coanpared with Earth, (density of water=19] 
Donati’s; Oct. 5th, | seurdesoes t0.t00 3:8 
“ Oct. 6th, T3585. 806.000 raves Vl 
Pe Oct ath: 1] sepatvaws z, 075 
First Comet of 1811, B27,945.000 382 13°6 
Halley’s Comet, B90,593,000 rs 12°6 
Comet of 1799 FETT PIO DIT 7335 63 
Comet of 1 807, T.13s.8ss 000 320s ta ES, 
eee; Oct. bib, | erve dee wus S358 v gehts 9 | 
rcs (Oct: 8th, | ow eetbe eee ot ae 5%. 
The ee although differing considerably among them- 
es, an 1 
Matter, like the earth, more or less covered with water, of which 
© greater portion is ordinarily in the condition of ice. 
_ lt the masses, as above determined, are too large to be admis- 
sible, we must then draw the theoretical inference that the mat- — 
ter expelled from the nucleus derives part of its velocity from a 
orce of projection. That such a force of projection is in fact 
Sometimes in operation, if the present theory be true, may be 
Seen from the results given in the following table. The calcula- 
Hons are here made for the outer envelope; for which the values 
. 
of NV were 821,500 miles and 20,000 mile. ‘i 
| dj Mass compard) Density. | 
Se mates obi ered | with Earth. [(density of water=1) 
aay met, 1811, Oct, 6th, 3 4 68°5 
;? ? ; Ies,5$s, 000] rs r : 
Donati’ 1858, Oct. 2d, haere Z136 18°7 
i 
ae Similar indication is afforded by the apparent increase in 
® mass of Donati’s comet, from Oet. 5th to Oct. 6th, and 8th, 
48 given in the former table. 
Pa 
