14 REPORT—1850. 
parts of the wood; at the base of the twisted branches, but nothing was observed 
which could determine the cause or the chemical effects of the electrical agent*. 
On Isoclinal Magnetic Lines in Yorkshire. By Professor Puicries, FLR.S. 
The author stated, that about fifteen years sitice, in the course of some experi- 
mental researches on terrestrial magnetism, his attention was caught by the appa- 
rently deep flexures of the isoclinal lines in Yorkshire, flexures certainly independent 
of local magnetic polarities. As a general inference from his inquiries, he suggested 
the dependence of these flexures on the physical configuration of the country, the 
isoclinals advancing northward on and toward the hills, and retiring southward in 
the valleys. (See Brit: Assoc. Reports for 1836, p. 51.) 
The observations on which this conclusion was based, were made by means of in- 
struments which the author had himself constructed. For verification of these and 
other results, he procured, in 1837, an excellent six-inch dip circle of Robinson, and 
has now obtained an additional set of determinations with this new instrument, 
which may be confidently trusted, with careful manipulation and in magnetic weather 
not unfavourable, to one minute of a degree or less. 
The results, being collected either by combination into five lines nearly parallel to 
the magnetic meridian, or into groups which represent separately the elevated and 
depressed portions of the surface, agree with the inferences which were presented to 
the Association in 1836; the isoclinals retiring southward in the vale of York, and 
advancing northward both on the eastern and the western hills. 
The author showed the general probability of this result from other sources of 
evidence, remarking on the fact, that in plain and evencountries the /ocal isoclinals were 
parallel to or deviated but little from the general isoclinals obtained by the method 
of least squares, while in the hilly districts, as the Cumbrian tract, North Wales, 
South Wales, and the mountain tracts of Ireland, the local isoclinals were much but 
still systematically bent from their general direction, and sometimes (as between 
Criffel and Skiddaw) crowded together in a singular manner. ‘ ¥ 
He noticed as desirable, for the complete reduction of delicate observations of this 
hature, a set of careful measures on the‘diurnal variation of dip. His own researches 
on this point indicated a single daily progression, with a maximum at 9 A.M., mean 
at 3 P.M., and minimum at 9p.m. The hours however, and the amount of differ- 
ence from maximum to minimum, appeared subject to much fluctuation. On an 
average about three minutes appeared to be the difference (in summier) between 
~ thaximum and minimum. s, 
Possibly the deduction of this variation, by resolving the horizontal and vertical 
forces in the direction of total force, would be preferable. [The late researches of 
Mr. A. Brown, by whomi the periodical variations of the dip had been traced at 
Makerstotin, were here referred to.} The dip at York appeared to be, oh an average 
of thirteen years, diminishing about 23 ina year. The author proposed to increase 
the nuniber of stations to fifty before submitting the results to a final and rigorous 
computation. 
On the Refractive Indices y several Substances. 
By the Rev. Prof. Bapen PoweEL1, F.R.S. Se. 
Having on former occasions endeavoured to extend the list of observed indices for 
the standard rays of the solar spectrum given by prisms of different media; by means 
of an apparatus described, along with the statement of the results, in my report to 
the British Association, 1839, I now beg to offer to the Association the indices in like 
manner obtained for the four following media. The rare oil of spikenard I received 
throtigh a friend from the late Mr. Hatchett, by whom it was carefully prepared 
perfectly pure; for the other three Iam indebted to Mr. Nevil Story Maskelyne: 
The results in each case are the meats of several repetitions. In two instances (the 
oils of lavendar and sandal-wood), the absorption of the violet rays (as in 80 many 
* Since the Report was presented, Mr. Wauchope has cleared a larger portion of the roots, 
and has found them split and blackened considerably. 
