80 Observations respecting Halley* s Comet. [Jan, 



credit is due, that we are enabled to predict its place as has 

 been done in the present instance to a few (six or seven) 

 days. Should there be an individual now in existence who 

 can doubt for a moment the truth of universal gravitation, 

 (on which theory, this as well as every other astronomical 

 prediction is built ) it will be as well to shut the book for 

 ever, for nothing further or more conclusive in the way of 

 argument can be urged. Not however to exhibit an impa- 

 tience which would ill become the defenders of truth o§ the 

 sublimest nature, it may not be amiss here to explain why 

 the astronomer Halley when calculating upon the same 

 grounds as the astronomer of modern days, could not have 

 predicted to an equal degree of accuracy ; and to notice the 

 causes which have given rise to an error of eight or nine days 

 in the present return — to pursue such a question through all 

 its minutiae would be at once to set down and write a volume ; 

 it will therefore be necessary to handle the enquiry rudely, 

 and state off hand, that in the days of Halley the existence 

 of five of the Planets composing the solar system (Georgian, 

 Juno, Ceres, Vesta, and Pallas were unknown. Since this 

 time likewise, the Comets of Biela and Encke have been 

 recognized as forming a part of our system and several others 

 which are conjectured likewise to belong to it have been 

 observed. Now the effect of every one of these is, conti- 

 nually to draw the Comet from its path and to disturb one 

 another ; the amount of the perturbation varying with the 

 time, inversely as the square of the distance, and directly as 

 the weight of the disturbing body; had Halley predicted 

 the present return of the Comet, and should it have turned 

 out that he was two or three months in error, so far from 

 throwing any discredit upon his theory (ignorant as he was of 

 the causes now enumerated) it would on the contrary tend 

 much to confirm it ; moreover in the days of Halley astro- 

 nomical instruments were rare, and their construction of the 

 rudest possible kind when compared with those of the present 

 day, insomuch so that comparatively little or nothing could 

 be known with regard to the relative weight of the Planets; 

 now in the prediction of Halley of the return of this Comet 

 in 1682, he took account only of the action of the planet; 



