transactions of the sections. 13 



Electuicitt. 



On the Electrical and Mechaniail Properties of Hooper's India-riihher Insulated 



Wire. By W. Hoopek. 



The autlior at a previous Meeting described the method by which he secures the 

 durability of iadia-rubber. Diagrams representing the effects of pressure and im- 

 mersion were shown, from which it was seen that pressure improves the insulation 

 of his wire in tlie same way as is observed with gutta percha. The result of care- 

 fully-conducted experiments, extending over three years, proves that the absorption 

 of water is so small that the most refined electrical tests failed to discover it. 



On the DejpolnrizatioH of Iron Ships, to prevent the Deviation of the Compass. 



By E. Hopkins, €.E. 



Extract of a Letter from Senhor Capexlo, of the Observatory, Lisbon, on 

 Maynetie Disturbance, to Balfouk Stewart, of the Kew Observatory. 



The author sent three Tables representing graphically tlie most important 

 residts deduced from the curves of our magnetographs for the year 1864. lie had 

 followed the plan of General Sabine in separating the greatest disturbances of the 

 three elements. Thus he had considered as a disturbance of the declination every 

 ordinate which diflered from the monthly mean by 2' -3 or upwards; while the 

 separating value for the horizontal force was -0011 of the whole horizontal force, 

 and that for the vertical force '00032 of the whole vertical force. The instruments 

 were at work during the whole of the year 1864 ; and of the 8760 hourly observa- 

 tions of each instrument, the observers only failed in measuring 97 for the declina- 

 tion, 139 for the horizontal force, and 159 for the vertical-force instrument. The 

 number of disturbances have been — 



For the declination 1043 



For the horizontal force 810 



For the vertical force 982 



From a diagram exhibited, giving the hourly variations yearly and half-yearly of 

 the three elements, it was seen that the progress of the declination for each period 

 is very regidar. The mean daily range of declination during the six months from 

 April to September, while the sun is north of the equator, is 9'-20; while during 

 the six months from October to March, when the sun is south of the equator, this 

 range is less, being barely 6'. For the dip, the corresponding curves are much 

 disturbed from 6 p.m. to midnight, especially for the six months when the sun is 

 north of the equator. The total force gives a well-pronounced minimum at 11 a.m. 

 during the six summer months, and ll'' 30"" a.m. during the six winter months. 

 The daily range is gTcatest for the six summer mouths, and least for the six winter 

 months. 



The diagi'am of disturbances gives for the declination a maximum of the westerly 

 distm-bances at about 8 a.m., and a minimum about 10 in the evening. On the 

 other hand, the maximum of easterly distiu'bances is about 10 in the evening, and 

 the minimum about 6 in the morning. 



The curves for the horizontal-force disturbances are irregidar. The maximum 

 of disturbances tending to increase the horizontal force takes place about noon, 

 while the minimmn is about 1 a.m. But here one is much struck with the great 

 disproportion between the distm-bances tending to increase and those tending to 

 diminish the horizontal force, the latter being both the most numerous and the 

 gi'eatest in amount. The maximum and minimum of these latter disturbances take 

 place a little later than the maximum and minimimi of the disturbances tending to 

 increase the force. 



With respect to the vertical force, the curve of disturbances tending to increase 

 this element resembles to some extent the curve of easterly disturbances, or disturb- 

 ances tending to diminish the westerly declination. In this same diagram blue and 



