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THE CIVIL ENGINEER AND ARCHITECTS JOURNAL. 



[April, 



IMPROVEMENT OF GEODESICAL INSTRUMENTS. 



As connected with the profession of a civil engineer, the principles 

 and practice of surveying is an important branch of study ; under 

 this term, we do not confine our meaning to the delineation of a few 

 fields, or of a long narrow slip of land, such as railway plans usually 

 present, but to the survey of extensive tracts of country or of is- 

 lands, usually denominated trigonometrical surveying, because that 

 branch of mathematics known as plain and spherical trigonometry, 

 forms the basis or principle upon which such surveys are carried on. 

 This branch of surveying has hitherto been principally confined to the 

 officers of the corps of royal engineers, by whom the trigonometrical 

 survey of the united kingdoms has been for so many years conducted; 

 but although too much unheeded by students in civil engineering, it 

 forms a legitimate branch of their studies, otherwise they cannot be 

 said to be eligible to occupy such important stations as surveyors- 

 general of our colonial possessions, where such surveys are so much 

 needed, nay almost indispensable. Connected with this subject, the 

 knowledge of the use and adjustments of the higher class of geo- 

 desical instruments is necessary, such as theodolites of large dimen- 

 sions, but more, especially an instrument called the altitude and azi- 

 muth instrument, from its two-fold application to the measurement of 

 vertical and horizontal or azimuthal angles, and its peculiar applica- 

 bility to the purposes of so much of astronomy as is requisite in such 

 class of surveying, or we may call it the instrumental link which con- 

 nects astronomy with geodesy and geography; any improvements, 

 therefore, in the construction of instruments thus connected with the 

 profession we profess to advocate, come fully within the limits of our 

 province, and therefore we are desirous of calling attention to the 

 following extracts from a paper that has been read before the Royal 

 Astronomical Society on January 13th last, entitled " On a new ar- 

 rangement of a Vertical Collimator attached to the Altitude and Azi- 

 muth Instrument." By William Simms, Esq. 



" The only essential respect in which the altitude and azimuth in- 

 strument now before the society ' differs from similar instruments by 

 which it has been preceded, is this. The azimuth or vertical axis is 

 perforated and fitted with an achromatic object-glass having a dia- 

 phragm in its focus, so as to serve, in conjunction with the spirit-level 

 upon the instrument, as a vertical collimator. 



•' At present the spider lines in the diaphragm of the collimator 

 form an acute cross, subtending an angle of about 30 c ; but the pre- 

 ference of this arrangement over two parallel lines placed very nearly 

 together, so as to present a narrow space for bisection, admits perhaps 

 of question ; my own habit being that of bisecting au angle by 

 Leads me to give to it the preference, although 1 have tound by ex- 

 periment that very satisfactory results may be obtained by the other 

 arrangement. 



" In this state of things, if the telescope be directed vertically 

 downwards, the image of the cross in the collimator will be seen 

 u|»'ii the diaphragm of the telescope; and the adjustment, indepen- 

 dently of verticality, which must be effected by the spirit levels at- 

 tached to the instrument, consists in so rectifying the optical axes, 

 that the centies, or intersecting points in the telescope and collimator, 

 remain coincident during an azimuthal revolution. 



" The mode of adjustment, described in order, may be as follows, 

 admitting, however, of variation at the pleasure of the observer. 



" 1. I; will be found convenient that the instrument be first generally 

 levelled: — the azimuth axis by turning 180' in azimuth, and correcting 

 by the feet screws of the tripod and the adjusting screws of the 

 spirit levels; but, in all cases, it the error be not beyond the range of 

 t ! '- -rales, it is far better to leave these screws untouched, and to 

 apply the correction by reference to the divisions upon the scales. 

 r I be axis of the altitude circle must be rectified by the striding level, 

 exactly in the same manner as in the transit instrument ; all which, 

 however, is too well understood to need a particular description, in 

 :i'ace. 

 " 2. To adjust the line of collimation, bring the vernier marked a 

 to 90 c or 27u c upon the azimuth circle, and, by means of the ad- 

 justing screws at the eye end of the telescope, make the middle ver- 

 or meridian line, bisect the angles of the collimator cross, turn 

 J8<>^ in azimute, and correct half the error by the above-mentioned 

 screws, and the remaining half by moving the object-glass of the col- 

 limator. 



" 3. To correct the nadir point, set the vernier a to 0° or 180° 

 upon the azimuth circle, 90° distant from its former position, and 

 make the middle horizontal line bisect the angles of the collimator 



1 An instrument with the collimator attached was exhibited in the 

 meeting room ot the Society. 



cross; turn 180° in azimuth, and correct half the error by giving 

 motion to the telescope by means of the tangent screw, and half by 

 moving the object glass of the collimator. The micrometers should 

 now be set to the zero points upon the altitude circle. By those, 

 however, who prefer numerical corrections to mechanical adjustments, 

 which, when extreme accuracy is aimed at, are always tedious and 

 difficult to execute satisfactorily, the nadir point may be readily de- 

 termined by reading the altitude micrometers with the circle in the 

 reversed position. The judications of the spirit-level fixed to the 

 micrometer bar must, of course, be carefully attended to in such a 

 determination. 



" I shall conclude this notice by observing that the new application 

 of the collimator does not deprive it of any uses and conveniences 

 which it has in any other form, while at the same time it possesses 

 advantages peculiar to itself. Its property, in common with the ver- 

 tical floating collimator, of enabling the observer to set the axis of the 

 altitude circle perfectly horizontal, irrespective of the riding level, is 

 one which the ievel collimator does not possess. In common with the 

 others, however, it affords a ready means of verifying and correcting 

 the essential adjustments of the instrument without reference to any 

 external object. An object adapted for such purposes should be both 

 distant and well defined, conditions which imply a clearness of atmos- 

 phere perhaps not generally met with in any climate, and much less 

 in that of our own country ; moreover the collimator is equally avail- 

 able by night and by day: the light of a small lamp or taper being 

 sufficient to render the lines visible. 



" But it has greatly the superiority, particularly in operations out 

 of doors, over the vertical floating, and also over the level collimator, 

 because the latter requires supports independent of, and equally 

 steady with, that upon which the instrument itself is placed; things 

 by no means easy of attainment under any circumstances, and to the 

 scientific traveller often perfectly impracticable. 



" Neither is it a small advantage to dispense altogether with an 

 additional instrument, which, to say the least, lessens the number of 

 the traveller's cases, and with them his cares also. An extra instru- 

 ment may by accident be injured, or through forgetfulness left behind, 

 or for want of time to set up, or a steady support when set up, prove 

 useless when it is most needed. But, by the new arrangement, the 

 collimator becomes part and parcel of the instrument itself, and is so 

 completely protected from injury that an accident could hardly impair 

 or destroy it without at the same time destroying the entire instru- 

 ment. Its introduction, too, into the perforated axis adds so little to 

 the original cost of an instrument, that it may make a final claim on 

 the score of economy." 



THE VELOCITY OF WATER IN VERTICAL PIPES. 



We have been favoured with another letter from our able corre- 

 spondent T. F N, in reference to our position that water flows 



down vertical pipes with only half the velocity due to the height 

 when issuing from an orifice. Our correspondent is not convinced by 

 our arguments, and he reiterates his former opinion supported by 

 many illustrations, and by the grave authority of Belidor. The same 

 fault, however, which we before complained of, pervades the whole 

 of his present argument; he takes for granted the question in dispute, 

 and assumes, as established data, the very points respecting which 

 we are at variance. We will let him, however, speak for himself. 



" Sir — I am sure that, if all your readers derived as much useful 

 information and pleasing instruction, as I am bound to say I did, from 

 your remarks in the last month's Journal, on the ' Velocity of water 

 in vertical pipes,' conveyed in a style at once clear and copious, they 

 must look forward anxiously for the fulfilment of the promise iu the 

 concluding passage, of returning at some future period to a subject 

 deserving so much consideration and study. Before you do so, how- 

 ever, I am anxious to point out to you what I conceive to be an error 

 in your reasoning, and, at the same time, uphold the assertions con- 

 tained in my last letter on the subject. — 1st. That the velocity of the 

 column of water in a vertical pipe maintained full, is not half that due 

 to the height, but that it is expressed by the same formula as for all 

 other falling bodies. — 2nd. That cohesion does not satisfactorily ex- 

 plain the continuous flow of water in a vertical pipe; in other words, 

 that there would be a continuous flow in the pipe even if one particle 

 did not cohere to that immediately above it. 



" It may appear presumptuous in me to endeavour to reply to your 

 remarks, and to maintain these positions after reading your able ar« 



