TRANSACTIONS OF SECTION A. 497 



according to which it depends generally in part on the influence experienced some 

 time before the time of observation, and not wholly on the influence at the time, 

 seems to have been first definitely noticed and discussed scientifically by Sir 

 Edward Sabine, in his analysis of the results of the magnetic observations in the 

 Antarctic exploring expedition of Sir James Eoss, in the Erebus and Terror 

 in the years 1839-48. 



The practical rule and caution given above is of great importance in the navi- 

 gation of iron ships. The amount of the error which may be found cannot be pre- 

 dicted for ships in general, nor for any particular ship, except after much experience 

 and careful observation. A small effect of two or three degrees,* such as that 

 referred to in the Admiralty Manual as found in M. Gaussin's experience, may be 

 observed in the course of quietly swinging a ship by hawsers or steam tugs. If the 

 ship under weigh is steamed round on the different courses, the amount of the 

 " Gaussin error " may generally be greater than if she is hauled round by warps ; 

 but we must not be sure that it will be so, because the shake of the screw, which 

 enhances the magnetization on the east or west courses, may shake it out again be- 

 fore the observation is made on the north or south courses. A good practical rule 

 in correcting the compass is, after having got it quite correct on the north and 

 south course, correct just half the error which is found after that on the south or 

 north course in the regular swinging of the ship. 



The warning at the head of this article is particularly important for ships of 

 war after firing guns when on easterly or westerly courses, if the course is then 

 changed to north or south, and particularly if after the firing the change of course 

 is effected under canvas, without the shaking of the ship's magnetism produced by 

 the engines and screw. 



The warning is also very important for ships steaming through the Mediter- 

 ranean eastwards or westwards, and then turning south, through the Suez Canal 

 or north round Cape St. Vincent ; and for ships steaming eastward from America, 

 and then turning northwards or southwards into St. George's Channel. 



11. On the Electrical Properties of Bees' Wax and Lead Chloride. 

 By Professors J. Perry and W. E. Atrton. 



Professor Ayrton commenced by noting the close way in which investigations 

 in the various branches of physical science were linked one with another, and by 

 remarking that experiments on electric absorption ought to have no less interest for 

 the scientific engineer than those on the increasing strain of materials under con- 

 stant mechanical stress had for the electrician. He next explained how, in conse- 

 quence of the absorbed charge in water being immeasurably greater than the 

 surface charge, the direct method of determining experimentally the specific induc- 

 tive capacity employed by Mr. Perry and himself, in their experiments on ' Ice as 

 an Electrolyte,' failed to give the result equal to the square of the index of refrac- 

 tion for light of infinitely long waves, and he suggested that the method recently 

 employed by Mr. Gordon for measuring the specific inductive capacity of solids 

 with very rapidly reversed charges might possibly, if applied to water, give an 

 answer approximately more equal to the square of the index of refraction ; how- 

 ever, he was inclined to think that, since Mr. Gordon's method for solids °-ave 

 (after the application of the proper correcting factor for the thickness of the 

 dielectric) numbers closely agreeing with the received specific inductive capacities, 

 there existed no known method for correctly ascertaining the electric capacity of a 

 liquid. 



For although a condenser might be made of opposed metallic plates separated 

 by a space almost entirely filled with a liquid dielectric, which did not, however, 

 touch either plate, and although, according to the ordinary nomenclature, the two 

 plates in such an arrangement would be said to be insulated from the water and 



* Much greater effects than this are actually found in the cases of gun practice 

 and of long steaming on easterly or westerly courses referred to below. 

 1878J K K 



