468 OPPOSITION OF MARS, 1849-'50, 



was overcast. Mr. Childe has been directed to observe the star again with the transit instru- 

 ment, which will clear up the discrepancy. There are several stars in the locality. 



NOTE. When the telescope is west of the pier, and in the meridian, vernier 1 of the hour 

 circle should read Oh. Om. Os. On turning the telescope westward, the readings increase. The 

 circle is numbered from Oh. to 247*. 



BAROMETEKS. 



(1.) The barometer employed in the equatorial room is by Newman, and nearly new. It is 

 of the "mountain" construction, and is in excellent order. The frame or case is composed of 

 bronzed brass, quadrilateral shaped above the cistern. The vernier and scale are of silvered 

 brass : the former is screwed to the frame ; the latter is worked by a rack and pinion. The 

 diameter of the tube is not given, but it appears to be two-tenths of an inch. The ratio of its 

 area ( T \) to the area of the cistern is marked on the case. 



(2.) The standard barometer in the circle room, by Jones, No. 580, is the same which Mr. 

 Henderson has described in the Memoirs of the Astronomical Society. Its frame is of brass, and 

 its cistern is adjustable. This barometer is always employed with the mural circle room obser- 

 vations. 



(3.) Newman's was hung upon the mural circle pier on two occasions for several days, and 

 compared with Jones's while the height of the column ranged between 29. 724 and 30.128 inches ; 

 the mean of the temperatures common to both being about 69, (lowest 67. 2 ; highest 70. 8.) 

 Jones's cistern was carefully adjusted for each comparison. Newman's is not adjustable. 



Thirty-two comparisons gave as follows : 



From 29.724 inches to 29.8 inches, 8 comparisons, mean =: 0.0102 inch. 



29.8 ' to 29.9 "5 " " = 0.0088 " 



29.9 " to 30.0 " 7 " " = 0.0100 " 

 30.0 " to 30.128 " 12 " " =0.0123 " 



The mean of the sum of these means is 0.0103 inch. 



The arithmetical mean of the whole is 0.0106 " 



by which the indications of Newman's are lower than the standard by Jones. Now the variation 

 or capacity correction for 0.402 inch should be 402 X .; <f = 0.008, whereas by experiment it is 

 only 0.002. It is singular that the neutral point is not marked upon Newman's. The lower end 

 of the scale is hid, but I am not aware of any contrivance by which it could operate on the 

 height of the mercury in the cistern. Perhaps the safest course is to adopt 0.010 inch for 

 the common difference, particularly for the present observations on Mars. 



(4.) The correction for capillarity (.029) is common to both. 



(5.) For connecting Newman and the Cape standard (Jones's) with the standard of the Royal 

 Observatory, Greenwich, the Royal Society's Daniel may be referred to ; but this requires an 

 investigation which will be apparent from what follows. 



(6.) Mr. Henderson states, (R. Ast. Soc. Mem., vol. x, p. 51:) "For the purpose of ob- 

 taining comparisons of the observatory barometer with the Royal Society's standard, Sir John 

 Herschel had the kindness to lend an excellent mountain barometer, made by Troughton. This 

 was compared with the Royal Society's standard by Mr. Hudson ; first in January, 1832, before 

 I went to the Cape, when it was found that both barometers being at the same temperature, and 

 the Royal Society's being corrected for the difference of capacities of the tube and cistern, and 

 for capillarity, Sir John Herschel's barometer was 0.048 inch lower than the Royal Society's ; 

 and again in October, 1833, after my return, when it was found in like manner that Sir John 

 Herschel's barometer was 0.050 inch lower than the Royal Society's. From all the compari- 

 sons made at the Cape it appeared that Sir John Herschel's barometer was 0.057 inch 



lower than Jones's. Hence, it is inferred that Jones's is 0.008 inch higher than the 



Royal Society's standard. These differences have been applied to the readings of the barom- 



