262 



NATURE 



[January 12, 1905 



from year to year were closely alike. In horizontal force 

 the annual changes recorded at the two stations did not 

 agree so closely, and on the average the change at Falmouth 

 was somewhat the greater. 



Whilst the mean daily range of temperature at Falmouth 

 — a seaside station — is notably less than at Kew, the daily 

 ranges of declination at the two places are as nearly as 

 possible equal, and the daily range of horizontal force is 

 somewhat larger at Falmouth. 



The annual variation of diurnal temperature range is 

 again notably less at Falmouth than at Kew, the winter 

 range at the former station being relatively high, and the 

 summer range low. There is in this case a somewhat 

 analogous state of matters in magnetics, the difference 

 between the diurnal ranges at midsummer and midwinter 

 being relatively less at Falmouth than at Kew. 



Analysing the diurnal inequality of temperature into 

 harmonic terms, General Strachey (Phil. Trans, for 1893) 

 found that the local time of occurrence of the maxima was 

 distinctly earlier at Kew than at Falmouth, the difference 

 being greatest for the 24-hour term, for which it amounted 

 to nearly an hour. When the declination and horizontal 

 force diurnal inequalities are similarly analysed, the local 

 times of occurrence of the maxima are so nearly alike at 

 the two stations that it is impossible to say with certainty 

 which is the earlier. This result applies to the average 

 year of a sun-spot cycle. 



When the annual variations in the amplitudes of the daily 

 ranges in declination and horizontal force at Kew, and of 

 the 24, 12 and 8-hour terms in the diurnal inequality, were 

 expressed as Fourier series, with an annual and a semi- 

 annual term, there proved to be a remarkably close agree- 

 ment between the dates of occurrence of maximum in the 

 annual terms, and also in those of the semi-annual terms 

 for the several elements. The same phenomenon appears 

 at Falmouth, and there proves, moreover, to be a remark- 

 ably close agreement between corresponding Kew and 

 Falmouth dates. This result again applies to the average 

 year of a sun-spot cycle. 



Applying Wolf's formula R = a-)-fcS, associating the range 

 R of a magnetic element with sun-spot frequency S, results 

 are obtained for the variation of b and b/a throughout the 

 year at Falmouth very similar in character to those 

 previously obtained for Kew. 



Taking the above formula, but making S represent in 

 turn the areas of whole sun-spots, umbrse and faculee as 

 given by the Astronomer Royal, values are calculated for 

 a and b in the case when R represents the range of declin- 

 ation or horizontal force in the mean diurnal inequality for 

 the year. A comparison is then instituted between the 

 ranges for individual years of tlje 12-year period as calcu- 

 lated from the values of a and b thus found, and the 

 Astronomer Royal's mean yearly data on the one hand, 

 and as actually observed on the other. When S represents 

 areas of whole sun-spots or of umbrae, the agreement 

 between observed and calculated ranges is nearly though 

 not quite so good, especially in horizontal force, as when 

 S represents Wolfer's sun-spot frequencies ; but when S 

 represents areas of facul^ the agreement is much inferior, 

 especially in years of sun-spot maximum. 



" The Effect of Temperature on the Thermal Conduc- 

 tivities of some Electrical Insulators." By Dr. Charles H. 

 Lees. 



The substance the thermal conductivity of which is to 

 be determined has the form of a cylinder about 8 cm. long, 

 2 cm. diameter, surrounded by a thin cylinder of brass and 

 placed in a Dewar tube. The heat is supplied by the passage 

 of an electrical current through a platinoid wire embedded 

 in the substance parallel to the axis of the cylinder, and 

 about 04 cm. distant from it. The temperature is measured 

 by the electrical resistance of two short spirals of No. 40 

 pure platinum wire, down the centre of one of which the 

 heating wire passes. 



The difference of temperature of the two spirals is deter- 

 mined by making them two arms of a resistance bridge, 

 the other two arms of which are equal. By means of 

 mercury cups resistances may be placed in series with either 

 of the spirals until a balance is obtained. 



A few values of the conductivities in C.G.S. units for a 



NO. 1837, VOL. 71] 



portion of the range of temperature on the hydrogen scale 

 are given in the following table : — 



At 120° abs. At 180' abs. At 240' abs. 



00058 ... 0-0052 



o-ooii .. 000091 



O-OO0S6 . 0-00070. 



O-OOOS5 ... 000070- 



O-O0O&2 .. 0-00076- 



0-00065 ... 000061 



0-00065 - 000063. 



000054 ... 000052 



Geological Society, December 21, 1904. — Dr. J. E. Marr. 

 F.R.S., president, in the chair. — On certain genera and 

 species of Lytoceratida; : S. S. Buckman. This paper deals, 

 with certain specimens sent by Mr. Beeby Thompson from 

 the Northampton Sands, one of which is remarkable for its 

 homoeomorphy with Phylloceras. — The Leicester earth- 

 quakes of August 4, 1893, and June 21, 1904: Dr. C. 

 Davison. The earthquake of 1893 was a twin, vi'ith its- 

 principal epicentre between Markfield and Woodhouse 

 Eaves, and the other near Tugby, about seventeen miles- 

 to E. 34° S. Its disturbed area contains about 2200 square 

 miles. On June 21, 1904, two shocks were felt : the first, 

 a very slight one, at about 3.30 a.m., the second at 5.28 a.m. 

 The epicentre of the earlier shock was in the neighbourhood! 

 of Markfield and Groby, or near the south-eastern margin 

 of the north-western focus of 1893. The distance between 

 the epicentres of the earthquakes of 1904 was about twelve 

 miles. Thus the foci of 1904 appear to have occupied the 

 nearer margins of the foci of 1S93. — The Derby earthquakes, 

 of July 3, 1904 : Dr. C. Davison. Although weaker than 

 the earthquake of March 24, 1903, this shock, owing to its- 

 occurrence at 3.21 on a Sunday afternoon, was felt over 

 a much wider area (about 25,000 square miles). As in 1903, 

 the earthquake was a twin, the epicentres being almost 

 exactly coincident with those of that year, one being situated 

 near Ashbourne, and the other, about six or seven miles 

 from it, near Wirksworth and Matlock Bath. — Twin-earth- 

 quakes : Dr. C. Davison. In a twin-earthquake, the shock 

 consists of two maxima of intensity, or of two distinct parts 

 separated by a brief interval of rest and quiet. In Great 

 Britain, one in every twenty earthquakes is a twin, and 

 our strongest shocks (the Colchester earthquake of 1884, 

 the Hereford earthquake of 1896, &c.) belong to the same 

 class. The phenomena show that twin-earthquakes cannot 

 be caused by reflection or refraction of the earth-waves, or 

 by the separation of the waves of direct and transverse 

 vibrations, or by the repetition of the impulse within the 

 same or an overlapping focus. They must therefore be 

 due to impulses in two detached, or practically detached, 

 foci ; and it is shown that all the known phenomena of 

 twin-earthquakes can be thus accounted for. In British 

 twin-earthquakes, the distance between the epicentres varies 

 from four to twenty-three miles, the average for seven recent 

 earthquakes being. between ten and eleven miles. As a rule, 

 the foci are elongated approximately in the direction of the 

 line joining them, showing that they are portions of the 

 same fault. The foci appear to be situated at different 

 depths, and, in two cases, the fault probably changes hade 

 in the region between them. 



Royal Microscopical Society, December 21, 1904. — 

 Mr. G. C. Karop in the chair. — Mr. Conrady read a short 

 paper explaining an experiment he exhibited to prove the 

 phase-reversal in the second spectrum from a grating of 

 broad slits, the mathematical proof of which he gave in 

 his paper on theories of microscopical vision read before 

 the society at its last meeting. The object consisted of 

 two gratings one above the other, similar in every respect 

 except that one had broad slits and the other had narrow 

 slits. In accordance with what was theoretically predicted 

 by the author, the difference was brought out when the 

 direct light plus the first and second spectra of one side 

 were admitted, but when the direct light w-as cut off by 

 the movement of a shutter the image of the broad slits 

 underwent a startling change. The lines jumped across to 

 positions mid-way between the correct ones, showing there 

 was an antagonism of phase between the light of the first 

 and that of the second spectrum. .Some photographs show- 

 ing the effects produced by cutting out the various spectra. 



