313 



TRANSIT, OR TRANSIT INSTRUMENT. 



TRANSIT, OR TRANSIT INSTRUMENT. 



sil 



the instrument is reversed will be . The time which a star takes j 

 to pass between the two circles, or the correction, is equal to the i 



error of collimation 



th(J supposition we j^^ made) ia _ 



cos. declin. of star 



* + 0-114 sec. declin. 111. end West. 



0-114 sec. declin. 111. end east. 

 The sign changes for stars tub polo. 



Recently a method has been devised for recording transits, founded 

 upon the principles of electro magnetism. The sidereal clock is made 

 to break the electric circuit at regular intervals of one second, and by 

 a process similar to that employed in Morse's telegraph, the effect is 

 impressed on a recording apparatus in connection with the clock. The 

 observer is also enabled to break the circuit at any instant between 

 two successive beats of the clock, and to record the fact in its proper 

 place upon the registering paper. Hence in determining the transit 

 of a star, the observer breaks the circuit at the instant of the star's 

 passing each successive wire, and the results are imprinted in their 

 proper place upon the recording apparatus. The distance between one 

 of such recorded results and the nearest second as imprinted by the 

 regular break-circuit apparatus of the clock, will indicate the fractional 

 part of a second corresponding to the instant of the star's passage of 

 the wire. In this method the eye and the sense of touch are called 

 into operation instead of the eye and ear, as in the usual method. It 

 originated in the United States of America, where it has been practised 

 since the year 1849. It has been subsequently adopted at the Royal 

 Observatory, Greenwich, and also at Altona, on the Continent. The 

 differences depending on personal equation are almost annihilated by 

 this method, which is well adapted for recording the transits of a great 

 number of stars within a short compass of time. 



The next adjustment is to make the axis horizontal. If the poles of 

 motion are in the horizon, the great circle which the instrument, freed 

 from collimation, describes, must pass through the zenith. Put on the 

 level, and bring the bubble into the middle. Now rock it a little to 

 and from the observer, and see whether the bubble still remains in the 

 same place. If, in pushing the level from you, the bubble runs towards 

 the left hand, this shows that the level tube itself is set askew upon 

 its support, and that the left end, being nearest the observer, is elevated 

 by that motion above the right end. Screw the small screw seen at m, 

 and release its antagonist (these are piithiny screws) until a considerable 

 rocking motion scarcely moves the bubble at all. There is generally a 

 cross level, to show when the principal level is upright ; and this 

 - should be brought to have its bubble in the middle, when the principal 

 level has been adjusted as above. The fork in the present example 

 serves the same purpose when the level itself has been carefully 

 adjusted. To level the axu, bring the bubble of the level to the same 

 reading at each end (the numeration of the division is supposed to 

 begin from the centre) by the elevating screw at the left hand Y. 

 Reverse the level and bring the ends again to the same reading, half by 

 the elevating screw of the Y, and half by the two screws seen at n, 

 which raise or depress the level tube in its supports. On returning 

 the level to its first position, the bubble should still be in the centre ; 

 but if not, it must be brought there, half by the Y elevating screw and 

 half by its peculiar screws ; and the operation must be repeated till 

 this is effected, that is, if the observer cannot or will not calculate the 

 effect of a small error, which may easily be measured. If he can (and 

 there are few observers at present who cannot), the process is pretty 

 much as follows ; the graduation being supposed to be unite each equal 

 to 15" : 



Illuminated end West. Telescope South. Altitude 45'. 

 Observer North. 



Level. 



East end of bubble, 4'7ti. West end, 5'84. 



The_ level is now reversed end for end, and the two ends again read 

 off: 



East end . 5-24 West end 5-34 



Mean, East 5-00 West . 6'-59 



which is the reading which the level would show in both positions if 

 it were in adjustment. Hence the west end is higher than the east by 

 half t the difference, or 0-295. The level should be applied in reversed 

 positions several times, and a mean taken. 



Now let the telescope itself be reversed, and suppose the following 

 entries of observations to be made : 



Illuminated end East. Telescope North. Altitude 45. 



Observer North. 



East 4-93 West 5-66 



5 -42 5 -19 



The difference is now 0"25, and the west end is consequently too 

 high by half the difference, or by 0-125, a result which diners from 

 the former (Illuminated end West) by 0"17. If the partial observa- 

 tions have been pretty accordant (we suppose 0-17 to be the mean 

 result of a considerable number of observations), this difference between 

 the values of the inclination, according to the position of the illumi- 

 nated end, must be supposed to be owing to a difference in the pivots ; 

 and if so, a little consideration will show that to obtain the true incli- 

 nation of the axis in the two positions, \ of 0-1 7 must be subtracted 

 from the level error 111. end West, and must be added to the error 111. 

 end East. The true level errors therefore are 



111. West, + 0-295 -0'-042, or + 0-253; 

 111. East, -f 0-125 + 0-042, or + 0-167. 



By the mean of a great many careful observations made when the 

 temperature is steady * and the sky overcast, the difference of the 

 pivots, if it exists, is to be ascertained, and the correction due to that 

 cause is to be applied to the indication of the level. 



The error of inclination in the axis being measured, the corresponding 

 correction which is to be applied to the observations is thus computed : 

 If the west end of the transit axis be raised, it is clear that the circle 

 perpendicular to that axis will continue to cut the.horizon at the north 

 and south points, but will pass to the east of the zenith, from which it 

 will be removed by an arc equal to the inclination of the transit axis. 

 All the stars above the pole will therefore appear to pass too early, and 

 those below the pole will pass too late ; and if the inclination be 15", 

 the effect in time upon any star will be 



cos zenith distance 

 cos declination 



xl- 



Now the level graduated as we have described gives the inclination 

 in parts of which 15" is the unity; hence the corrections to be added 

 to the observed times of passage of stars will be, using the previous 

 example- 

 cos, zen. dist. 

 111. end West +0'253'x 



Mean East 5 -175 



Mean West 5 '425 



The rotation of the earth requires tho apparent place of a star to be 

 increased by 0-0206. con. latitude X oc. of ' declination. Thu come to the 

 umc thing as subtracting 0-0206 x cos. latitude from the collimation cor. 

 rection. In Imt. 41 JO 1 this = 0-013. The corrections for collimation become 



+ 0-101 xec. dec. HI. end West. 

 0-H7xsec. dec. * 111. end Eart. 

 t If the unit were 30", the west end would be higher by the whole difference. 



cos. zen. dist. 

 111. end East + 0-167' x C03 declin of star. 



The above corrections, for errors of collimation and inclination, are 

 purely instrumental, and, as the reader will perceive, do not require 

 any celestial observation. Before proceeding to the third adjustment, 

 that by which the great vertical circle now described by the telescope 

 is made to pass through the pole, it will be proper to mention how 

 transit observations are actually made, and then describe how this 

 error is corrected or computed. The instrument is in or very near the 

 meridian ; a star on entering the field is placed between the two hori- 

 zontal wires. It will then gradually travel through the field, describing 

 a parallel to the horizontal wires, and passing over the vertical wires in 

 succession. The observer looks at his clock a little before the star 

 cornea to the first vertical wire, and counting the beats steadily forward 

 by ear, determines as well as he can the second and decimal of a second 

 at which the star is immediately under the wire. He writes this down, 

 counting all the time, and goes through the same process at each of the 

 seven wires with which his instrument is furnished. When the star 

 has passed all the wires, he looks again at the clock to see that his 

 count is right, and then sets down the hour and minute corresponding 

 to the last wire. The habit of mentally counting on to sixty while 

 writing down the observation is easily acquired. The estimation of 

 the decimal of a second at which the star is covered by the wire is a 

 matter of more difficulty, and, with some persons, requires considerable 

 practice. \ The observer is to attempt to fix in his mind the places of 

 the star with respect to the wire at the preceding and succeeding beat, 

 and to divide the second in proportion to the two spaces. Thus if at 

 16* the star is rather nearer the wire, before passing, than it is at 17', 

 after passing, he sets down 16-4 : if he judges the proportion to be 

 less than one to two, he sets down 16"3 ; and so on. Finally, the 

 mean of the observations over the seven wires is to be taken, which is 

 to be used as the actual time of transit. It would be desirable that an 

 observer should begin by learning to note the observation with con- 

 siderable exactness, and if it may be, under the care of a practised 

 guide ; but the observation is so simple, that every one acquires the 

 power who has the will to try ; and we believe that, with a little 

 experience, one observer is nearly as good as another. 



* By adding the two readings together, you have tho length of the bubble, 

 which, if tho temperature is steady, will continue to be of the same value 

 during the series, but will grow ahorter by heat. The level should be exposed 

 some time before it is used. The coherence in the values of the length is a 

 proof of the goodness of the observations. If the bubble continue to be of the 

 same length when the zero is changed it is probable that the curvature is 

 uniform. After the level has taken the temperature of the air, about a minute 

 should be given for it to settle after each application, but not more than two. 

 The observer should learn to read the level rapidly and boldly, as the zero 

 changes if a light is held near to it for many seconds. 



t Success will depend a good deal on the definition and magnifying power 

 of the telescope, and also on the sharpness of the wires and the beat of the 

 clock. The advantage of a distinct audible beat is very often entirely over- 

 looked by clock-makers who are nut observers. 



