542 PHILOSOPHICAL TRANSACTIONS. [aNNO J 768. 



that when these kind of obsen'ations are taken, not too near the horizon, when 

 proper stars offer for this purpose, and the whole skilfully managed ; these kinds 

 of observations fall but little short of those taken immediately in the meridian. 

 He cannot therefore hesitate to recommend, that the quartile observations be 

 taken out of the meridian, as well as in it : in the first place, by Dollond's 

 micrometer, if stars offer in proper positions; and if rot, 2dly, by taking dif- 

 ferences of right ascension and declination between the planet and the stars, by 

 the common micrometer, in case proper stars offer themselves for this purpose : 

 but as it frequently happens, that no proper stnrs offer themselves to micrometers 

 of either kind , and this is still more likely to happen in the obfcrvations of 

 Venus, which will be chiefly in the day light ; he offers (what to him is) a new 

 method of observation out of the meridian ; and which, though he esteems it 

 not equal to micrometer observations of either kind, he apprehends will fall so 

 little short of it, and prove so much superior to any other method now in prac- 

 tice in these cases, he gives this general method of observing out of the 

 meridian, byway of appendix. 



In the next observation of Mars, it has been stated, that in the meridian ob^ 

 servations alone, the menstrual parallax, according to the smallest estimation, 

 maybe expected to amount to 31' in longitude; which, turned into right ascen- 

 sion, will make about 1^ of time : now if it may be allowed, that a well-practised 

 observer can take the time of a transit to 4 part of a second, over a single wire, 

 if he has 3 wires, or more, as usual, the mean of the 3 should be within -^ part 

 of a second ; or within -^ part of the whole quantity in question : it is however 

 a matter of chance, whether the mean of 3 may or may not be within 4- part of 

 the whole ; and as ■ equal errors may be committed in the observations of the 

 transits of the stars, with which the right ascensions of the planets in question 

 are compared ; which it is an equal chance whether they tend to correct or in- 

 crease the errors committed in the former ; yet if, as has already been proposed, 

 the obser>?ations be continued for 2 or 3 months, instead of one ; and obser- 

 vations, taken the day preceding and subsequent to the days of conjunction, 

 quartile, and opposition ; and this as well out of the meridian as in it ; we can 

 hardly doubt but that, if the weather should favour, so many cheques would be 

 formed, that, from the next opposition of Mars alone, the affair may be brought 

 within a 24th part of the whole ; and if to this be added the force of such deter- 

 minations, as may be drawn from observations on Venus, before and after her 

 transit over the sun next year, it can hardly be doubted, but that those 3 will 

 bring us within a single second of a degree, subtended from the nearest planet ; 

 and these conclusions will be further strengthened by future observations ; as 

 2 years will scarcely pass without affording one opportunity or more of this kind. 



As Mr. S. meant not to embarrass himself with exact computations, he has 



