and of the Altitude and Azimuth Instrument. 1 1 1 



indeed, the imperfect manner in wliicli its ndjustment is per- 

 formed, together with its weak frame, renders it altogether 

 unfit for that purpose. A is a transit, and as perfect as the 

 telescope which it bears will allow. This instrument, ni the 

 observatory, is certainly the most important of all, and as a 

 portable one, the best, 'and attended with the least trouble for 

 keeping the rate of a chronometer. Add to this, that by 

 taking the difference of right ascension between the moon and 

 stars," it affords perhaps the yery best means of ascertammg 

 the difference of longitude between distant places*. 



For observing equal altitudes, both instruments artf very 

 good : and I would only give the preference to A as being 

 more likely to preserve the position of the telescope unvaried, 

 by keeping in adjustments better during the requisite length 



of tune. '^^ c 



For taking altitudes at a distance from the meridian tor 

 finding the tune, as much may be said in favour of A : yet for 

 this purpose, upon the whole, R seems to have the advantage ; 

 because these variable alti-'udes may be taken by repetition, 

 and the errors of division, to which A is liable, almost done 

 away. Ready means of phicing an instrument in the plane of 

 the meridian *are of the utmost importance to the tiavelhng 

 astronomer, and that by corresponduig azimuths is perhaps 

 the best. A is not only peculiarly adapted to this method, 

 but also furnishes the best means of doing the same thing by 

 all the other ways. When R has a good azimuth circle, its 

 ■plane may be brought into the meridian by the same means, 

 at least near enough for its own purpose, for it cannot with 

 propriety be regarded as a meridian instrument at all. 



Respecting the dimensions of the two circles under com- 

 parison, I have, considering every thin^, thought that 18 inches 

 should be tlie greatest diameter for R : this admits ot a two 

 feet telescope, and gives sufficient room for the screws tor 

 clamping and slow motion to pass each other, and to be con- 

 veniently handled. The pressure of heavy telescopes upon the 

 centre-work is certainly detrimental, however tlcsirable good 

 ones may be ; and were it not li)r want of finger room in a 

 12 ind/one, notwiUistanding the diminutive telescope winch 

 it bears, I would give the preference to the latter dmiension. 

 The instrument A may safely be extended to a diameter ot 

 two feet, beyond which "it could not justly be called portable; 

 but besides' this, there are other reasons lor not carrying it 

 much further than this limit: and 1 admire the courage more 

 than the sound judgement of those who have constructed very 

 * Many writers liavc given an erroneous rule Cor this purpose : sec Piiil. 

 Mag. vol. XV. I). 97. j.^j.^.g 



