TRANSACTIONS OF THE SECTIONS. 
19 
jUKof the instrument. As zero of the first wheel, or what is the same thing, the 
poiater B advances one graduation from the 101st division of tlie second wheel for 
rerlOOrevoIntioDS of the screw; so for every 10,100 revolutions of the screw, the 
lOlit division of the second wheel, or what is the same thing, the pointer C, falls one 
pidoation apart from the 102nd division of the third wheel. 
Ihe author adds notices of the applicability of this instrument to a variety of cases 
Btiidtrevolatioiis arc to be counted, work summed, or distances measured. 
Sur hs Cometes Periodiques de Lexcl de Faye et de Vico. 
By Prof. Leverrikr. 
^Aamniof Observations made at tise expense q/" General Sir T. M. 
Brisbane, to determine the variations of the laws of Terrestrial Miif/netism 
rtspai to h&yht in the Atmcsjdiere. By J. A. Broun. 
lie highest of the range of the Cheviot hills is about eighteen ihIIcb to the E.S.E. 
'^Geoccal Sir T. M. Brisbane’s ohaervatorj’ at Makerstoun, the height of the aura- 
Mteingsctio feet above the level of the sea, and 2440 feet above the level of the 
The proximity of this hill was suggestive of several experimental re- 
in terrestrial magnevlsm which could only be well pcrfoimed with the aid 
if* fixed observatory. A series of obaervations was accordingly made in the bc- 
of June, having two objects in view 
tint. The determination of the variation of the total force of the earth's magnet- 
Mivithhel^t. For this purpose observations were mode at Makerstoun and on 
njiaiiiitof Chc-viot with a theodolite magudnmeter by Jones, for the absolute 
intcusity, and with an incliuometer by Robinson, for the variations of 
J'KdiuI, The determination of the diurnal variations of the declination and of the 
component of intensity at different heights, 
f- Broun had as yet been able to oxanjine only n portion of the observations, 
"““dy- those having reference to the variations of ine magnetic declination, 
inri precautions adopted need nut be detailed at present ; it may be 
BoaKt however that observations of the declinometers were made on Cheviot 
Wi , 
llto Bobservations strictly simultaneous, two chronnmetein belonging to Sir 
Brisbane were taken to Cheviot, and these were compared each morning 
„ ®e*un shone, with the observatory clock at Makerstoun (uighteon milce di- 
*'1 a method it was believed now for such a purpose. Mr. Welsh, the prm- 
• Makerstoun, threw the reflexion of the sun from a common mirror 
station, at times previously agreed on ; when the sunshine was n<^ 
fcnifc f the very brilliant reflexion was seen with great distinctness with 
, Weye, at other times it could be seen with the aid of a tele.scopc. Tlie re- 
. ^ interccptwl by a screen at SO” 0*. 35’“ O’, 40”' O’, &c. Makerstoun mean 
X, T? of the chrunometere were thus obtained with great accuracy. 
Mbit» j ’ Itad also occurred to Sir Thomas Brisbane, might be lound 
of longitude.] Gunpowder flashes were also obtained near 
deduced from a comparison of the observations at 
confined himself at present to two, which were eixhibi e 
^.^'^distmctness. ‘ . 
•to b. .i.*^®“®“wwterlv declination occurred earlier at the highest station; tJiat 
bflje greatest westerly position of the north cn<l of the magnet was attained 
1 m tk mrridinn iiossage on Cheviot than at Makerstoun. 
^^ •itfonrange or the amount of the diurnal oscillation was le.'ss at the 
[I!mI confinned by the observations of different days. 
reference to the theory of the dmmal rariations. 
were made at the same time. A comparison with t ese 
C2 
