67 



ternal objects by the same principle on which the mariner steers by 

 his compass. Thus the two cranial eyes are analogous, in principle 

 and situation, to two magnetic compasses placed upon a ship's deck ; 

 while the third, or cerebral eye, corresponds to another compass 

 placed in the cabin below; and the mind, situated like the captain- 

 mariner in his cabin, knows, from consulting the cerebral eye, on 

 what point of direction the body is steering ; although the mind no 

 more perceives either any external object, nor yet any image in the 

 cranial eye, than the mariner perceives (even in the vulgar sense 

 of the wo'rd perceiving) the far-off land, or haven, towards which he 

 is surely making his way." 



A paper was read, " On the Thermostat or Heat Governor, a 

 self-acting physical Apparatus for regulating Temperature;" con- 

 structed by Andrew Ure, M.D., F.R.S. 



The principle of the instrument here described is the unequal 

 expansion of different metals by heat. A bar of zinc, alloyed with 

 four or five per cent, of copper, and one of tin, about an inch in 

 breadth, one quarter of an inch thick, and two feet long, is firmly 

 and closely riveted along its face to the face of a similar bar of 

 steel of about one third in thickness. The product of the rigidity 

 and strength should be nearly the same, so that the texture of each 

 may pretty equally resist the strains of flexure. Twelve such com- 

 pound bars are united in pairs by a hinge joint at each of their ends; 

 having the zinc or alloy bars fronting one another. At ordinary 

 temperatures these bars will be parallel, and nearly in contact; but 

 when heated, they bend outwards, receding from each other at their 

 middle parts, like two bows tied together at their ends. When a 

 more considerable expansion is wanted, a series of such bars is laid 

 one over the other. The movement thus resulting is applied by the 

 author in various ways to regulate the opening of dampers, letting 

 in either cold air or cold water, or closing the draught of a fire- 

 place, as the case may be. He proposes its employment to regu- 

 late the safety valves of steam boilers, as working with more cer- 

 tainty than the common expedients. 



A paper was read, " On the Determination of the Thickness of 

 solid Substances, not otherwise measurable, by Magnetic Devia- 

 tions." By the Rev. William Scoresby, F.R.S. Lond. and Edin. 

 Corresponding Member of the Royal Academy of Sciences of Paris, 

 &c. 



In the first part of this paper, the author states the results of a 

 series of experiments undertaken by him with the view of ascertain- 

 ing whether all bodies are equally and uniformly permeable to the 

 magnetic influence. Out of a great number of substances not ferrugi- 

 nous, but of various qualities, thickness, and solidity, which were 

 subjected to trial, no instance occurred of their offering any per- 

 ceptible obstruction to the action of a magnet on a compass, when 

 interposed between them. No interruption to this action occurred 

 even when the intervening bodies were iron ores, of which several 



