S21 



TRANSIT, OR TRANSIT INSTRUMENT. 



TRANSIT, OR TRANSIT INSTRUMENT. 



noise, and the clock has a low beat, it is found necessary to have a 

 second clock called a journeyman, which strikes loudly and speaks as 

 it were for the transit clock. The observer makes them beat pretty 

 nearly together, and then listening at the principal clock and noting 

 the difference, he either pushes forward or delays the pendulum of the 

 journeyman to make the coincidence perfect, and this ought to be 

 continued until he cannot distinguish between the two beats when 

 standing close to the transit clock. Let a person try to make this 

 coincidence by looking at the transit clock and listening to the journey- 

 man, and if he can, or can very nearly do this, it is evident that he 

 notes an appearance at the time it happens. Perhaps by trying the 

 game thing when fatigued, he might detect a change in his perceptions, 

 for the coincidence of sounds, as judged of when equalised by standing 

 near the weaker source, is one in which a tolerable ear can scarcely be 

 more than 0-01 or 0"02 out at farthest. 



The position of the horizontal axis has been all along supposed to be 

 measured by the level, and this is certainly the most ready method. 

 But the level may happen to be broken, or, unless it comes from a very 

 careful maker, it may be sluggish, or unequally divided. The beautiful 

 levels which accompany Ertel's instruments, which are covered at the 

 ends with parchment and filled with ether, are very liable to leak, as 

 we know by experience. In such a difficulty, our celebrated surveyor 

 Captain W. F. Owen raised a tall pole, and having put thereupon a 

 distinct mark, adjusted his instrument by moving the elevation screw 

 until the wire passed through the mark seen directly and by reflexion. 

 In another instance, where the level was broken, an observer of some 

 name was unable to supply ita place, and a projected set of observa- 

 tions failed in consequence. The simplest method is that pursued by 

 Captain Owen, substituting the pole star or other slow-moving star at 

 its culmination for the tall pole. When the axis is thus nearly cor- 

 rected, which may easily be done when the star passes the first wire, it 

 is better to observe the star over the rest of the wires half directly and 

 half by reflexion, and to reduce each set to the mean wire. On draw- 

 ing the figure it will be seen that any error of level will affect the 

 transit of a star seen directly one way, just as much as it will affect 

 the transit of the same star, seen by reflexion, the other way, or that 

 the difference of the two transits, after each set has been reduced to 

 the mean wire, is twice the error due to inclination : that is, the differ- 

 ence of the transits in the two positions is, when the star is above the 



pole = ^ ' x t, from which i is determined, and may be used 



r | 5 



for all the other observations. The observation will succeed very well 

 with any slow-moving star, if the observer has time to shift from one 

 position to the other without hurry ; or he may use two high stars, 

 each observed over all the wires, if they have the same altitude, or if 

 he should happen to know the other errors of his instrument. Indeed, 

 if he has no objection to solve simple equations with four unknown 

 quantities, he may proceed exactly as we have shown in former 

 instances, introducing another term with t and its coefficient, and 

 changing the sign for the observations by reflexion. 



Observations by reflexion of Polaris are well suited for another 

 purpose, namely, for examining the value of the level scale by means 

 proper to the instrument itself. Raise the west end until the bubble 

 is nearly at the west end of the scale, and by a mean of half a dozen 

 readings, reversing each time, ascertain the error of inclination. Now 

 observe Polaris exactly as we have before mentioned, or, if the observer 

 likes better, directly over the 1st, 2nd, 6th, and 7th wires, and by 

 reflexion over the 3rd, 4th, and 5th ; reduce each set to the mean wire, 

 and calculate by the formula already given the true inclination of the 

 axis. On a following night repeat the operation, the illuminated end 

 being on the same pier, only lowering the west end of the axis until 

 the bubble is nearly at the east end of the wale, and get as before two 

 values for the inclination, one from the scale, and another by observa- 

 tion. Take a mean, and you will have the true value of the parts.of 

 the scale. If the result varies much in the two experiments, it shows 

 either that the curvature of the level is unequal, or that one pivot is 

 thicker than the other. This may be ascertained by the level alone, 

 as we have shown above, or would be indicated by a difference between 

 the direct and reflected observations when the axis is horizontal by the 

 level, or by comparing the inclination obtained from reflexion in the 

 manner last pointed out, in reversed positions of the instrument, 

 supposing the I's not to change during the experiment. Thus if the 

 inclination be determined by observing Polaris over the first half of 

 the wires directly and the second half by reflexion, a value of the 

 inclination will be found. Reverse the instrument, and make the same 

 observations upon another slow-moving star, and you will have a 

 second value of the inclination, which should agree with the former if 

 the pivot* are equal ; half the difference, if it exists, is the difference in 

 the radii of the pivots. The level however affords a much easier, and, 

 we believe, better measure of inequality ; but it will not show if the 

 pivots be elliptic, which the observations by reflexion would do if 

 rtars at different altitudes were observed. If the two tests agree, it is 

 a reason for believing that the pivots are round within the limits of 

 these very searching experiments. But as we believe these observation* 

 have never yet been made, it would be useless to expatiate further 

 upon their possible advantages. The late astronomer royal, Mr. Pond, 

 tented the transit at Greenwich by observing a set of stars directly and 

 a second set by reflexion after the axis had been most carefully 



ARTS AND 1CI. DIV. VOL. Till. 



levelled, and found that on reversing his sets on a subsequent night he 

 got the same mean interval, as he ought. Professor Woodhouse 

 examined his level scale by observing Polaris over half the wires with 

 one end high, and the other half with the other end high. This is less 

 sensitive than the method we have pointed out, but will do for its 

 purpose very well, if the instrument ia examined and verified by a 

 meridian mark between the first and second set of observations, other- 

 wise we should be afraid a change in azimuth might be caused by 

 turning the elevating screw, in spite of all the care of the artist to 

 prevent it. 



An eye-piece has been introduced into use within the last few years, 

 which, by illuminating the wires in a particular manner, enables the 

 observer, looking downwards into a basin of mercury, to see at the 

 same time the wires and their reflected image : if these be made to 

 coincide, the telescope is vertical, and therefore the axis horizontal. If 

 the micrometer wire be used to measure the interval, the result will 

 be found to be twice the inclination. 



The transit may be levelled, as it was in older times, by a plumb- 

 line, which, hanging from a frame placed close to the instrument and 

 in front of it, is made to pass over two dots, placed at the eye and 

 object end of the telescope. This is an accurate but intolerably trouble- 

 some method. In Groombridge's circle, Troughton used a plumb-line 

 hi a tube at right angles to the axis and to the telescope for the same 

 object. The images of the opposite dots at top and bottom were 

 thrown on the line by lenses, and viewed through microscopes, in the 

 way in which he always applied the plumb-line. We do not know 

 whether Mr. Groombridge adjusted the horizontal axis by means of 

 this plumb-line or no, but the artist himself said that he introduced 

 the tube principally to make the axis equally weak all round, finding 

 that it was previously so much stronger in one direction than another 

 as to give him trouble in dividing it. Finally, the axis may be 

 adjusted, or the inclination measured micrometrically, by means of a 

 vertical collimator, which is convenient enough, but, so far as our own 

 experience goes, rather uncertain in its indications, and much inferior 

 in both rapects to a good level. A really good level carefully and 

 frequently applied will show the position of the transit axis to about 

 0"'2 or 0"'3, or the inclination correction to 0"02, and this is a smaller 

 quantity than a considerable number of careful observations will 

 show. 



From what we have already said, it is evident that where exact time 

 is wanted, the collimation, inclination, and deviation factors are per- 

 petually required. The colliraation factor is merely a table of secants 

 of declination, and may be taken from any table of natural secants. 

 The inclination and deviation factors should be computed for each 

 observatory to every 10 7 of declination, and be tabulated for constant 

 use. For the stars often observed, we find it most convenient to have 

 a catalogue in which the log secants of decliriation, the natural secants 

 and tangents of decimation, and the factors for inclination and deviation, 

 are entered in parallel columns with the proper signs. The astronomer 

 royal employs a sliding-rule for these and similar computations. In 

 computing this table for a given latitude, the formulas will be advan- 

 tageously transformed thus : 



Inclination factor 



= COB $>+ sin <p tan S ; 



sin (<t> i) 

 Deviation factor = ~" = sm cos tan 5 : 



so that having the natural sine and cosine of latitude, and also the 

 log sine and cosine, the computation reduces itself to adding the log 

 tan. declination to these last. The necessary tables may be computed 

 in a few hours as far as is advisable ; for near the pole the change of 

 declination has so large an effect, that it is necessary to use the exact 

 declination. 



Great service would be done to amateur practical astronomers by a 

 judicious set of printed forms, in which to enter and reduce their 

 observations ; and by a set of tables sufficient for these small com- 

 putations, and not containing anything further. This can only be 

 obtained by repeated attempts, and after all most observers would 

 probably prefer a modification of some form, to adopting it implicitly. 

 We have tried to produce something in this way on which a better 

 attempt may be founded. The astronomer royal published a por- 

 tion of the forms used at Greenwich in the volume of the Observa- 

 tions for 1840, and we venture to recommend his practice to other 

 observers, in order that their less able brethren may profit by their 

 superior skill and experience. 



While he was employed in the Royal Observatory of Paris, Romer 

 proposed a method of determining the equinox by observing the azi- 

 muth of the sun at rising and setting near the time of the equinox, 

 which method he illustrated by an example. (' Basis Astronomic,' p. 

 107.) He thus got rid of the effects of parallax and refraction, and 

 deduced an accurate declination of the sun without an exact knowledge 

 of the latitude. The method is a very good one, though undoubtedly 

 inferior to that proposed and executed by Flamstced. Many years 

 after, Romer, on erecting a small observatory in his own country, 

 placed a transit east and west, that is, in the prime verticil. Almost 

 all his papers were destroyed by a great fire at Copenhagen, and it 

 does not seem by Horrebow's account that any use was made of this 

 prime vertical transit. He intended ,probably to observe the sun in 



