86 
Mr. herschel on the 
In general, the method of zenith diftances labours under 
the following confiderable difficulties. In the firft place, all 
thefe diftances, though they fhould not exceed a few degrees, 
are liable to refra&ions ; and I hope to be pardoned when I fay 
that the real quantities of thefe refractions, and their differences, 
the doctor had of the refult of his own obfervations with regard to the annual 
parallax, de la landf. only mentions “ M. erabley penfe que ii elle (la 
“ parallaxe) eut ete feulement de i" il l’auroit appercue dans le grand 
“ nombre d’obfervations qu’il avoit faites, furtout de y du Dragon.” But 
if ue all'o take in thofe lines upon which Dr. bradley feems to lay the 
greateft ftiefs, viz. “ I believe 1 may venture to fay, that in either of the 
“ tvvo ^ ars mentioned it does not amount to two feconds;” and if we 
allow for the magnitude of the liars upon which the obfervations were made, I 
think I have iaulv dated the full amount of all the a£tual proofs we have of the 
ffnallnefs of the annual parallax. Now, fmee rt has cfcaped the fined obfervations 
ol bradley, it is not likely that it fhould come up to the full quantity to which 
it n ight amount without being perceived ; and therefore the doftor might think 
it highly probable, “ that it is not fo great as one fingle fecond and his opi- 
nion, as well as de la lande’s, who believes it to be abfolutely infenfible, are 
perfectly confident with all the obfervations that have hitherto been made ; though 
the actual proofs , which are the fubjed of our prefent inquiry, do not extend fo 
far. Againd the parallax of Sirius de la lande (§ 2781.) mentions “ forty- 
■“ five meridian altitudes taken by Dr. bevis [a], with the eight-feet mural qua- 
“ drant of the Royal Obfervatory at Greenwich, none of which differed 3 or 4" 
“ from the mean altitude.” Now, if they differed 3 or 4" from the mean we 
may fuppofe they differed 6 or 8" from each other ; and that obfervations, fubject 
to fo many caufcs of error as I fhall prefently enumerate, and which differed fo 
much from each other, cannot give the lead evidence either for or againd a paral- 
lax, will need no proof. Refra&ion alone, which is liable to fuch changes at the 
meridian altitude -of Sirius, notvvithdanding the mod careful obfervations of the 
barometer and thermometer fhould be made to afeertain its quantity, would, 
with me, remain an unanfwerable argument againd the validity of fuch obferva- 
tions in a lubjed of this critical nicety. 
p] Thefe obfervations were not made by Dr. bevis, but extracted from the regifters of the Royal 
"Obfervatory at my defire, and calculated by myfelf, and lent in a letter by Dr, bevis to Paris. 
NEVlL MASK ELY NE. 
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