Fuly 2, 1885] 
NATURE 
215 
did not depend so much on high temperature as on pressure or 
the presence of water, and there is reason to believe that a tem- 
perature of about 600° to 800° F. would suffice to produce all or 
almost all the observed hydrothermal effects. For although in 
many instances of normal metamorphism new minerals are 
formed, the rocks are not fused, nor are the fossils destroyed. 
In Brittany, black slates which pass into schists with large 
crystals of chiastolite still show impressions of orthis, trilobites, 
and other Silurian fossils. Devonian strata in the Vosges pass 
into a rock consisting of pyroxene, garnet, epidote, &c., and yet 
vetain impressions of cov/s. 
Of the enormous tangential pressure exercised in the elevation 
of these chains, some idea may be formed when we consider the 
amount of compression which those portions of the crust have 
undergone. Thus, for example, Heim estimates that in the 
Alps the compression has been to the extent of 72 miles ; and 
in a recent paper by Prof. Claypole he arrives at the conclusion, 
after a careful investigation of the magnitude and width of each 
fold, that in the Appalachian Mounta ns ‘‘a tract of the earth’s 
surface, measuring originally 153 miles from south-east to north- 
west, has been so crushed and compressed that its present 
breadth along the line of section is only 65 miles,” and of this, 
in one part—the Cumberland Valley—‘‘ 95 miles of country 
have been compressed into 16 miles.” 
These vast compressions could not have taken place without 
the transformation into heat of the equivalent amount of me- 
chanical work, though the degree and centralisation of the heat 
would depend on the rapidity and completeness with which the 
crushing has been effected. It is not therefore surprising to find 
that, in some of the newer mountain-ranges, a small residual 
portion of the heat thus mechanically evolved may still exist and 
cause slight aberrations in the position of the underground is)- 
thermal lines, and the same cause may possibly account for other 
exceptional cases. 
The only sufficiently complete set of observations on a moun- 
_ tain-chain of this character that have yet been made are those 
before alluded to by Dr. Stapffin the St. Gothard Tunnel. The 
author has before given, in his paper on ‘‘ Underground Tem- 
peratures,” particulars of these observations, and therefore here 
only mentions that at the north end of the tunnel in the part 
where an axis of elevation of late geological age (Pliocene) 
traverses the range, the thermic gradient, which normally equals 
about 57 feet for 1° F., is there not more than 38 feet ; and for 
this Dr. Stapff states that there was no obvious explanation. 
The author concludes by expressing a belief that there exists, 
in the compression and motion of the strata which has always 
accompanied the upheaval of mountain-chains, a vera causa for 
the production of an amount of heat sufficient to produce one 
form of metamorphic action—a form which can affect only par- 
ticular regions—and he would, therefore, in order to show its 
distinctiveness from either contact or novmal metamorphism, 
designate it by the term of ‘‘ Regional Metamorphism.” 
Physical Society, June 13.—Prof. Guthrie, President, in 
the chair.—On the winding of voltmeters, by Profs. W. E. 
Ayrton and John Perry. As it is most important that volt- 
meters, ohmmeters, powermeters, and ergmeters should be so 
constructed that the percentage increase of resistance of their 
fine wire coils due to the heating effects of the currents passing 
through them should be as small as possible, the question arises 
as to whether such coils should be made of German silver wire, 
or of copper, or partly of German silver and partly of copper 
wire, and how the diameter of the wire should vary in different 
parts of the coil. The authors have therefore been led to in- 
vestigate the conditions that make this heating error a minimum 
with cylindrical coils of internal and external radii 7 and 74. 
At a place whose distance from the axis is ”, let the cross-section 
of the wire be ., p the specific resistance of the material ; then, 
assuming that 2 = 197%, p = po’, pr = po%p?*, and that a cur- 
rent, C, in one spire of radius 7 produces a magnetic effect, 
# Cr, on the supended needle, they find that the heating error 
is proportional to 
9 
LP n ryt ry" 
Yr, - 
oPo (Ae=7 2)? t= 7,% mm 
where f=d-a+ti,n=2+0-2a,m=2+1°71446 — qa. 
The conditions that make this expression a minimum are worked 
out in the paper, the result being that with one of their magni- 
fying spring solenoid instruments, where d = —1, the values of 
a and 0 giving a minimum value are a = 0°325 and 6 =~-o's, 
and since in practice 6 cannot be negative, they conclude tha 
6 =oand a = “4 give the best results—*.e., that all the wire 
employed in the bobbin should be of copper, and the law of 
increase of cross-section proceeding from the centre should be 
x = a,r°4. The actual waste of energy in the instruments is 
next considered, and, lastly, the authors show how to pass from 
a voltmeter with known winding, and whose maximum reading 
is P, to another of the same volume and shape whose maximum 
reading is to be Pj, and they conclude that, as they have shown 
that the waste of energy is the same in both for their maximum 
readings, the resistances of the instruments must be proportional 
to the squares of P, and Py, or, following the law already 
arrived at for a minimum error due to heating, the cross-sections 
of the wires of the two instruments at similar places must be 
inversely proportional to P, and P,. The employment of out- 
side coils for voltmeters is considered, and it is shown that if we 
desire the same error in the two instruments due to heating when 
the outside resistance coils are of the same size and shape, it is 
necessary to have the same ratio between the resistance of the 
resistance coi! and that of the magnetising coil in the two cases. 
To have a less or a greater error in the second case it is only 
necessary to use the equation— 
2+ FV 
T+ AV’ 
where / is a constant and V the volume of the German silver 
resistance coil. From this 7 may be determined and the ratio 
e(the error) = 
R “ . A so 
R of the resistances of the resistance coil and the magnetising 
a S 
coil is given by wt V, where D is a constant which, like 
ae. ee 
F, is obtained from experiments on the first instrument. The 
diminution of the heating error by using much iron in the instru- 
ment so as to obtain the same magnetic action with a much 
smaller current is discussed, and experiments were shown to 
illustrate how such employment of iron introduced a permanent 
magnetism error and caused the indications of such an instru- 
ment on the lower part of the scale to be uncertain and to depend 
upon whether measurements were being made with an increasing 
or a diminishing current.—On the manner in which light affects 
the resistance of selenium and sulphur cells, by Mr. Shelford 
Bidwell. In a communication made to the Society at its last 
meeting, the author had described a sulphur cell which behaved 
in all respects like a selenium cell when exposed to light. The 
action of this cell was supposed to be electrolytic, the sulphur 
containing a small quantity of sulphide of silver. If this were 
the case the result of a current traversing the cell would be to 
deposit sulphur upon the anode, and, as sulphur has an enormous 
resistance, that of the cell would increase unless the sulphur thus 
deposited combined with the silver. It is this combination that 
is believed to be much facilitated by light, a supposition the author 
believed he had confirmed by direct experiment. Mr. Bidwell 
had also measured the resistance of a piece of selenium that was 
believed never to have been heated in contact witha metal. The 
specimen was crystallised by heating for some time in a glass 
mould, two opposite sides cleaned, and two pieces of tinfoil 
between which the resistance was measured pressed against 
them, In this way the specific resistance was found to be 2500 
megohms, which is enormously higher than that of the selenium 
in the ‘‘cell,” a fact tending to confirm the theory that the 
conduction in such cells is due to the electrolysis of the selenides 
of the metals forming the terminal produced in the ‘‘ cooking,” 
and similar to that of the sulphur cell described above.—On the 
error involved in Prof. Quinke’s method of calculating surface 
tensions from the dimensions of flat drops and bubbles, by Mr. 
A. M. Worthington. In a series of well-known papers Prof. 
Quinke has recorded a large number of measures of flat drops 
and bubbles, from which he has deduced the values of tensions 
for the free surface of a liquid and for the common surface of two 
liquids in contact. The numerical results obtained in this way 
exceed those obtained from observations upon the rise in capil- 
lary tubes, which Prof. Quinke attributes chiefly to the fact that 
in the latter case the edge angle is not zero. Mr. Worthington, 
however, shows that the surface tensions obtained by Prof. 
Quinke with flat drops are too high, this arising from his having 
assumed that the drops were flat at the vertex. The error thus 
introduced is very considerable, amounting in most cases to as 
much as 10 per cent. of the whole value, and upon its being 
duly corrected, the values obtained do not appreciably exceed 
those obtained with capillary tubes. —On a comparison between 
the mercury standards of resistance issued by M. Mascart with 
those of the British Association, by Mr. R. T. Glazebrook. 
