438 Memorandum on the Construction [No. 138. 



be seen at a window in the house of the Catholic priest adjoining the 

 Church at Bettiah in the Chumparun district. A French gentleman, 

 (now Principal of the Dehli College,) acquainted me with the method 

 of laying down the meridian line from the above instrument, and I 

 have since then constructed several. All these meridians are of course 

 fixtures, but it occurred to me, that if a portable one could be made, it 

 would be extremely useful when travelling about the country in tents, 

 and an excellent substitute for one of Dolland's Universal Dials. 

 Though inferior to the latter in the circumstance of shewing only one 

 hour in the day ; viz. twelve o'clock, yet it would be superior in shewing 

 that hour with an accuracy unattainable by the Universal Dial, owing 

 to the variation of the compass by which the latter is always set. The 

 variation of almost every needle differs considerably, so that a know- 

 ledge of the general or average variation of the compass at a par- 

 ticular place, does not afford any information as to the variation 

 of the particular needle in your dial, and without ascertaining 

 this point, no dependence can be placed on the time given by the 

 dial. A variation of one degree makes a difference in time of 



THEODOLITES. about five minutes, and the annexed 

 Instrument. 



No. 4. 2° 38' 0" Variation East, memorandum, made by Lieut. Thuillier, 

 „ 16.2 21 January 1841 ' „ 



„ 12. 3 39 at Cuttack. (the Revenue Surveyor of Cuttack in 



91 9 37 fl 



," 23! 2 49 These needles 1841,) shews, that among a number of 

 ;; 79! 2 g 32 ft^ltloZ ^edles, all of the same length, some 



» 7 £< , l 36 22 differ from others to the extent of 4°; 



,, 5, 1 26 



supposing the latter to be the variation of the needle in an Universal 



Dial, there would be an error in the time of about 20 minutes. 



PRISMATIC COMPASSES. 3d. The Portable Meridian which I 



No. 2. 1° 15' 0" Variation East, am about to describe, if accurately con- 

 „ 3. I needles 2i . , . - . , . 



u 4, 4 00 inches long structed by professional instrument- 



" 34! 3 30 makers, such as Dolland, or Troughton 



" ia' o qq n and Simms, would give the time with 



„ 16. 3 30 much greater, if not perfect, accuracy, 



„ 119. 2 15 5 ' „ „ . . 



„ 23.1 15 as it would be free from all errors arising 



„ 22. 1 15 „ , . • r xu 



,, 40- 3 45 from the variation 01 the compass. 



C0 ^2 3 2 I) This instrument, shewing (or professing 



" 17* 1 45 'o to snew ) tne true meridian, would also 



