CIRCLE. 



489 



Cirtlt. Moreover, the three readings, when the angle measured 

 is the sextantal one (120) in a backward and forward 

 observation, (answering to right and left in the other 

 constructions), will just occupy the 720 degrees of 

 this circle. 



It might have been sooner noticed, that the degrees 

 of this circle are subdivided into three parts of 20' each, 

 and that each branch of the index subdivides to 20", the 

 verniers having 60 divisions, whose length is equal in ex- 

 tent to 59 of those upon the limb. 



The dark glasses, in sets of three, are fixed to the 

 instrument in such a manner, that any or all of them 

 may be used at the same time, as circumstances may re- 

 quire. This gives the circle before us a decided superio- 

 rity over all the others. Nor is it a matter of small im- 

 portance, with respect to convenience, that a handle is 

 found both above and below : By one or other of these, 

 in every possible position of the instrument that obser- 

 vation? can require, it may be held right or left, as is 

 found most convenient to the observer. It must, how- 

 ever, be acknowledged, that the reflecting circle is, un- 

 der the best circumstances, held two or three inches far- 

 ther from the observer than the sextant is, and, of course, 

 bears proportionally heavier upon his arm. Mr Trough- 

 ton is of opinion, that this inconvenience is the princi- 

 pal if not the sole cause, that the circle has not yet 

 come into general use ; but we would hope, that an in- 

 convenience so trifling, cannot deter a good officer from 

 exercising the best means of obtaining the truth ; espe- 

 cially, wnen we consider how much is entrusted to his 

 care, and that the safety of the whole depends, in a 

 great degree, upon ascertaining, in a long voyage, a 

 lew points of longitude and latitude, upon which, with 

 confidence, he can rely. 



We are given to understand, that our present astro- 

 nomer royal is a warm friend to the circle ; and that, in 

 the preface to the next publication of Nautical Alma- 

 nacks, he intends to recommend this instrument. From 

 the regard we have for the improvement of nautical 

 science, we sincerely wish that Mr Pond's endeavours to 

 extend the use of the reflecting circle may be as success- 

 ful as he can desire. 



The following instructions for adjusting and observing 

 with Troughton's reflecting circle, may be acceptable 

 to many of our readers. They are copied from a printed 

 paper, which is usually given with the instrument. 



Direction* 

 for tiling 

 Trough- 

 ton't re- 

 flecting 

 r rcle. 



Directioni for Observing with Troughton's Reflecting 

 Circle. 



" Prepare the instrument for observation by screwing 

 the telescope into its place, adjusting the drawer to fo- 

 cus, and the wires parallel to the plane, exactly as you 

 do with a sextant : also set the index forwards to the 

 rough distance of the sun and moon, or moon and star ; 

 and, holding the circle by the short handle, direct the 

 telescope to the fainter object, and make the contact in 

 the usual way. Now read off the decree, minute, and 

 second, by that branch of the index to which the tangent- 

 screw is attached ; also, the minute and second shewn by 

 the other two branches; these give the distance taken 

 on three different sextants ; but as yet, it is only to be 

 considered as half an observation : what remains to be 

 done, is to complete the whole circle, by measuring that 

 angle on the other three sextants. Therefore set the in- 

 dex backwards nearly to the same distance, and reverse 

 the plane of the instrument, by holding it by the oppo- 

 site handle, and make the contact as above, and read off 

 as before what is shewn on the three several branches of 



VOL. VI. J-ART II. 



the index. The mean of all six is the true apparent dis- 

 tance, corresponding to the mean of the two times at 

 which the observations were made. 



When the objects are seen very distinctly, so that no 

 doubt whatever remains about the contact in both sights 

 being perfect, the above may safely be relied on as a 

 complete set ; but if, from the haziness of the air, too 

 much motion, or any other cause, the observations have 

 been rendered doubtful, it will be advisable to make 

 more : and if, at such times, so many readings should be 

 deemed troublesome, six observations, and six readings^ 

 may be conducted in the manner following. Take three 

 successive sights forwards, exactly as is done with a sex- 

 tant j only take care to read them off on different branch- 

 es of the index : also make three observations backwards, 

 using the same caution ; a mean of these will be the dis- 

 tance required. When the number of sights taken for- 

 wards and backwards are unequal, a mean between the 

 means of those taken backwards, and those taken for- 

 wards, will be the true angle. 



It need hardly be mentioned, that the shades, or dark- 

 glasses, apply, like those of a sextant, for making the 

 objects nearly of the same brightness ; but it must be 

 insisted on, that the telescope should, on every occasion, 

 be raised or lowered, by its proper screw, for making 

 them perfectly so. 



The foregoing instructions for taking distances, apply 

 equally for taking altitudes by the sea or artificial hori- 

 zon, they being no more than distances taken in a verti- 

 cal plane. Meridian altitudes cannot, however, be taken 

 both backwards and forwards the same day, because 

 there is not time : all therefore that can be done is, to 

 observe the altitude one way, and use the index-error ; 

 but even here, you have a mean of that altitude, and 

 this error, taken on three different sextants. Both at 

 sea and land, where the observer is stationary, the me- 

 ridian altitude should be observed forwards one day, and 

 backwards the next, and so on alternately from day to 

 day ; the mean of latitudes, deduced severally from such 

 observations, will be the true latitude ; but in these, 

 there should be no application of index-error, for that 

 being constant, the result would in some measure be vi- 

 tiated thereby. 



When both the reflected and direct images require to 

 be darkened, as is the case when the sun's diameter is 

 measured, and when his altitude is taken with an artifi- 

 cial horizon, the attached dark-glasses ought not to be 

 used : instead of them, those which apply to the eye- 

 end of the telescope will answer much better ; the for- 

 mer having their errors magnified by the power of the 

 telescope, will, in proportion to this power, and those 

 errors, be less distinct than the latter. 



In taking distances, when the position does not vary 

 from the vertical above thirty or forty degrees, the han- 

 dles which are attached to the circle are generally most 

 conveniently used ; but in those which incline more to 

 the horizontal, that handle which screws into a cock on 

 one side, and into the crooked handle on the other, will 

 be found more applicable. 



When the crooked handle happens to be in the way of 

 reading one of the branches of the index, it must be re- 

 moved, for the time, by taking out the finger-ecrew, 

 which fastens it to the body of the circle. 



If it should happen that two of the readings agree 

 with each other very well, and the third differs from 

 them, the discordant one must not on any account be 

 omitted, but a fair mean must always be taken. 



It should be stated, that when the angle is about thir- 

 ty degrees, neither a distance of the sun and moon, nor 



