Dec. 



1874J 



NA TURE 



ence, and it was not until July 1873 that I received final 

 authority to order the necessary instruments. With the 

 sanction of the Secretary of State, I conferred with Mr. 

 Warren De la Rue on the subject, and am indebted to 

 that gentleman's zeal and experience for a great deal of 

 valuable assistance. 



The following is a list of instruments prepared by Mr. 

 De la Rue and myself, and sanctioned by Government : — 



I Photoheliograph. 



4 Equatoreal. 



I Altazimuth. 



I Transit Instrument. 



1 Chronograph. 



I Standard Clock. 



3 Journeyman Clocks. 

 I will briefly describe each of these. 



PItotoheliograph. — This is of the same identical size 

 and construction as those supplied to the stations 

 equipped under the auspices of the Astronomer Royal. 

 It is well known that these have been constructed from 

 the designs of Mr. De la Rue and under his close 

 personal supervision. I therefore advised that that 

 gentleman, as the first authority on the subject, should be 

 requested to superintend the provision of this particular 

 instrument, and that he should have carle blanclw for the 

 introduction of any improvements or additions suggested 

 by his constantly enlarging experience. My recommen- 

 dation was adopted, and I need hardly say that Mr. De 

 la Rue, with his usual public spirit, at once gave his 

 services to the Government and to science. This instru- 

 ment, like its prototypes, was made by Mr. Dallmeyer. 

 Janssen's apparatus, modified by Mr. De la Rue, for mul- 

 tiplying the photograms, has been supplied to this 

 instrument. 



Equatoreal. — This has an object-glass of six inches clear 

 aperture. It was made by Messrs. Cooke and Sons, of 

 York, and is generally of the form which those artists 

 have made familiar to the astronomical world. It is a 

 universal instrument, being capable of adjustment for any 

 latitude to 67° 39', and in either hemisphere, for which 

 latter purpose it has reversible driving- gear and two hour- 

 circles readily interchangeable. Besides a striding level 

 for making the declination axis horizontal, two delicate 

 levels are suspended from the centre of the telescope. A 

 graduated circle and delicate level is also attached to the 

 telescope near its eye end. This, in conjunction with a 

 micrometer in the eye-piece, will qualify the instrument 

 for determining latitude by the differential observation 

 of two stars of nearly equal zenith distances, north and 

 south. The instrument has two micrometers— a parallel 

 wire by the makers, and a double image micrometer by 

 Messrs. Troughton and Simms, precisely similar to those 

 supplied for other stations under the auspices of the 

 Astronomer Royal. The parallel wire micrometer has a 

 contrivance intended to enable the observer to record the 

 readings of the divided head without withdrawing his eye 

 from the eye-piece, as suggested by Mr. Christie, of the 

 Royal Observatory. The form actually adopted by the 

 makers differs both from that employed by Mr. Christie 

 and that prescribed by me. I am unable to express any 

 opinion on its efficiency, as it arrived from the makers 

 alter the instrument had been despatched to India, and I 

 could not try it. 



The instrument is well supplied with eye-pieces and all 

 necessary adjuncts, and the arrangements for bringing 

 down the various adjusting and slow-motion screws to 

 the observer's hand when observing are very complete, 

 and many of them, I believe, as novel as they are ingenious 

 and effective. 



I had but one brief opportunity of trying the object- 

 glass, but that sufficed to satisfy me that it is of a high 

 order of excellence. 



Allazimitth. — The place of this is supplied by the great 

 theodolite const; ucted, from my designs, by Messrs. 



Troughton and Simms, for the Great Trigonometrical 

 Survey of India, and now about to be employed for the 

 first time. The main features of this instrument were 

 described by me in a paper read before the Royal Society 

 and published in its Proceedings, No. 135, of 1872. I 

 may briefly state here that the horizontal circle is three 

 feet in diameter, read by five equidistant micrometers, the 

 circle being fixed and the micrometers revolving ; that 

 the vertical circle is two feet in diameter, read by either 

 two micrometers fixed, or by four capable of being shifted 

 so as to change the divisions and thus reduce errors of 

 graduation ; and that the telescope has an aperture of 

 3'25 in., and a focal length of 36 in., to which can be 

 attached either of two parallel wire micrometers — one for 

 measuring in the vertical, the other in the horizontal 

 plane, according to the class of observation, the wires 

 having both a dark and a bright field illumination. 



This is no doubt the most elaborate and most powerful 

 instrument of its class in existence. 



Transit Insirmnciit. — This is of a peculiar and, I 

 believe, in many respects novel form. It is from the 

 designs of Signer Magnaghi, of Genoa, modified by the 

 makers, Messrs. Cooke, of York, and myself Signor 

 Magnaghi's object was to produce an instrument capable 

 of determining latitude, as well as of performing the 

 usual functions of a transit instrument, those of deter- 

 mining time and longitude. The telescope has an aper- 

 ture of 3 in. and a focal length of 34J in. with a transit 

 axis of the usual form 1 8} in. in length between the pivot 

 shoulders. The stand is of cast iron, and consists of 

 first a massive circular base plate 24 in. in diameter, sup- 

 ported, according to the system introduced by me, by 

 three shakeless foot-screws, upon a single masonry pillar. 

 On the base plate revolves horizontally a second similar 

 plate with the two pillars cast hollow in one piece with it. 

 This upper revolving plate moves stiffly, and is provided 

 with slow-motion screws and four powerful clamping 

 bolts. When it is bolted to the lower base plate, tlie 

 two may be considered practically to be one mass. The 

 horizontal motion thus provided is used, first, for eflecting 

 the ordinary azimuthal adjustment of the instrument to 

 the plane of the meridian ; and secondly, the direction of 

 the meridian having been found, for placing the telescope 

 with great facility in the plane of the prime vertical for 

 the determination of latitude by that method. 



An apparatus is also supplied for lifting the telescope 

 and reversing its pivots on their bearings, the transit 

 axis level remaining suspended from the pivots during 

 the process. This arrangement also admits of the in- 

 strument being used to determine latitude by the diffe- 

 rential observation of two stars of nearly equal zenith 

 distances, north and south ; for which purpose the tele- 

 scope is provided with a parallel wire micrometer and a 

 delicate level. 



Chronograph.— Co\. Tennant attached, justly I think, 

 great importance to means being provided for the electric 

 record of the time observations which form so essential a 

 portion of the undertaking. He wished to have a chrono- 

 graph whicli should be capable of recording at the same 

 time, and without confusion, observations made with four 

 ditterent instruments, viz., the Photoheliograph, Equa- 

 toreal, Altazimuth, and Transit Instrument, and he indi- 

 cated the apparatus described by Lord Lindsay in Monthly 

 Notices, R.A.S. I tlierefore examined Lord Lindsay's 

 chronograph, then under construction by Messrs. Cooke, 

 of York, but, though no doubt suitable for a fixed obser- 

 vatory, such as his lordship is establishing, I thought it too 

 large and ponderous for the present service. I accor- 

 dingly arranged with Messrs. Cooke a much fighter and, 

 I may add, a less costly plan. By substituting continuous 

 bands of paper, similar to those used in telegraphic in- 

 struments, for sheets of paper carried by the large barrels 

 employed by Lord Lindsay, great increase of compact- 

 ness and lightness were secured. Four such bands are 



