136 



NATURE 



[June 9, 1898 



Magnetic Observations. 



Fortunately for the magnetic records secured at the observ- 

 atory, the proposed electric tram-line in the neighbourhood of the 

 Deptford Cattle Market has been successfully opposed. That 

 this would have seriously damaged the records there can be 

 absolutely no doubt, since it would have been only i\ miles 

 from the observatory : even now small agitations, due to the 

 running of trains on the South London Electric Railway, 4^ 

 miles from the observatory, can be clearly traced from the year 

 1890 on the horizontal and vertical force sheets, synchronising 

 with the disturbances in the earth current registers. 



The principal results for the magnetic elements for 1897 are 

 as follows : — 



Mean declination 16° 50' '4 West. 



Mean horizontal force by the Gibson \ 3*9877 (in British units). 



instrument in the library ... j I "8387 (in metric units). 



(67° 5''5 (by 9- inch needles)- 

 67°6''8 (by 6-inch needles). 

 67°7'-i (by 3-inch needles). 



These results are to a certain extent affected by the iron 

 in the new physical observatory and in the new altazimuth 

 pavilion. To eliminate this effect as far as circumstances would 

 allow, observations have been made during the past year on the 

 site selected for the new magnetic pavilion in Greenwich Park, 

 which is presumably free from any disturbing effect of iron. 

 The horizontal force has been observed monthly on this site 

 with the two deflection instruments (Gibson and Elliott), the 

 declination occasionally with the Elliott instrument, and a dip 

 with a Kew dip circle (Dover 74). 



. It appears from these observations that the declination at the 

 observatory has been increased by 3' to 4' through the intro- 

 duction of iron. 



. The mean horizontal force obtained with the Gibson instru- 

 ment in the park is I "8366 in metric units. In the same units 

 we have also the following differences : — 



Gibson in library — Gibson in park 

 Elliott in library — Elliott in park ... 

 Elliott in its usual position in library- 

 Elliott on Gibson pier 



Gibson in park — Elliott in park ... 



+ 0'002I 

 + 0-0084 



-f 00060 



+ o-ooio 



All the magnetic disturbances during 1897 were of a com- 

 paratively trifling nature. 



Meteorological Observations. 



The mean temperature of the year 1897 was 50° '3, being 

 o°'9 above the average for the fifty years 1841-1890. 



During the twelve months ending 1898 April 30, the highest 

 daily temperature in the shade recorded on the open stand was 

 90° '2 on June 24. The highest reading recorded in the 

 Stevenson screen was 87° -4 on the same day. The monthly 

 mean temperatures were in excess of their corresponding 

 averages in every month with the exception of May, September, 

 and March. In January the excess amounted to 5°, the mean 

 temperature for that month being 43° '6. In the preceding 

 fifty-seven years there is one instance only of a higher mean 

 temperature occurring in January, viz. in 1884, when it was 

 43° -9. A mean value equal to the present January value 

 (43° '6) was also recorded in two other years (1875 ^nd 1890). 

 The winter of 1897- 1898 was remarkably mild throughout, and 

 the temperature of the air fell to freezing point (or below) on 

 twenty-nine days only — ten of these occurring in March and 

 seven in December. The lowest temperature recorded during 

 the winter was 23° "3 on December 24. [The lowest temper- 

 ature recorded in January was 30° "O.] The mean temperature 

 for the five months 1897 October to 1898 February, was 44° '6, 

 being 2° '4 in excess of the average value. During the whole 

 period of fifty-seven years (1841 to 1897) this value has only 

 been exceeded three times, viz. in the winter of 1876-1877, 

 when the mean for the five months was 45° "8, in the winter of 

 1845-1846, when it was 44°'8, and in the winter of 1865-1866, 

 when it was 44° 7. A mean value of 44° '6 (the same as that 

 for the present year) was also recorded in the winter of 1848- 

 1849, and in that of 1868- 1869. 



The number of hours of bright sunshine recorded during the 

 twelvemonths ending 1898 April 30, by the Campbell-Stokes 

 instrument, was 1529 out of the 4454 hours during which the 

 sun was above the horizon, so that the mean proportion of sun- 



NO. 1493. VOL. 58] 



shine for the year was 0*343, constant sunshine being repre- 

 sented by I. 



An interesting comparison is made between the results as 

 given by the new and the old ball of the sunshine recorder for 

 1897. With the former 1542-6 hours were registered through- 

 out the year, while with the latter only 1268-4 hours, the excess 

 with the new ball amounting to 274*2 hours during the twelve 

 months. 



The rainfall for the year ending 1898 April 30 was 17-33. 

 inches, being 7*21 inches less than the fifty years' average. 

 The number of rainy days was only 149. This is a very small 

 annual rainfall ; the three smallest falls during the preceding 

 fifty years being 16-38 inches in 1864, 17-61 inches in 1867, 

 and 17-70 inches in 1858. 



Personnel. 



No change of any importance has been made with regard to 

 the staff during the past twelve months, Mr. Dyson continuing 

 to take special charge of the astronomical department, and Mr. 

 Cowell the astro-physical department, in which is included the 

 magnetic and meteorological branch. 



GUTTA PERCH A. 



T N a recent course of three lectures' delivered before the Society 

 -*- of Arts, and subsequently revised and reprinted from the 

 Journal of the Society, with additional illustrations and ap- 

 pendices in the form of a bulky pamphlet, Dr. Obach dealt very 

 fully with the history, origin, treatment and properties of gutt.i 

 percha. 



In the first lecture the early history, botanical derivation and 

 geographical distribution of this substance were related, and the 

 analyses of various commercial "brands," as well as exhaustive 

 statistics of the annual imports and exports of the material were 

 given. 



In the second lecture the mechanical cleaning processes and 

 chemical washing and hardening processes were described and 

 illustrated, and also the different methods of extraction of gutta 

 percha from removable parts of the trees, such as twigs and 

 leaves, explained. This lecture concluded with an enumeration 

 of the various natural substitutes for gutta percha which have 

 been proposed at various times, including the interesting material 

 known as balata. 



The third lecture dealt with the mechanical and electrica? 

 properties of gutta percha and its application for various technical 

 purposes, also its behaviour towards water, oxygen and ozone. 

 In conclusion the artificial substitutes for gutta percha were 

 briefly discussed. 



The following is a short report on those parts of the third 

 lecture which we think may be more especially interesting to the 

 readers of Nature. 



In order to simplify matters. Dr. Obach selected from the 

 numerous sorts of gutta percha which make their appearance on 

 the Singapore market twelve different "brands," which may be 

 considered as typical ; they are distinguished by the name of the 

 locality whence they are derived. For direct comparison and 

 easy reference these twelve materials were divided into four 

 groups, each group comprising materials more particularly related 

 to each other. The groups were designated as "Genuine," 

 " Soondie," "White," and "Mixed." 



It was explained that cleaned gutta percha consists essentially 

 of two constituents, viz. a hydrocarbon termed pure gutta (G) 

 having the composition CjoHjg, and being therefore isomeric 

 with oil of turpentine, and a resin (R) containing more or less 

 oxygen, and consisting principally of two substances named 

 Albane CjoHjgO, and Fluavile C.joHgjO. Besides these proxi- 

 mate components there is also a variable amount of extraneous 

 matter present in every commercial gutta percha, even after the 

 most scrupulous cleaning, which consists of finely-ground bark, 

 wood fibres, vegetable colouring matter, grit, &;c., summarily 

 termed dirt (D), and of water (W). 



Dr. Obach has found that the physical and mechanical pro- 

 perties of the various sorts of gutta percha depend almost 

 exclusively on the relative proportion of gutta and resin, 



i.e. the ratio — , whereas the electrical properties depend chiefly 



on the nature of the gutta and, to a lesser extent, upon that of the 



1 "Cantor Lectures on Gutta Percha," by Dr. Eugene F. A. Obach, 

 F.I.C., F.C.S., M.I.E.E. 



