1896.] Scientific Instruments used by the late Dr. Joule. 359 



temperature of the water in one of the vessels from that of the 

 maximum density, determines a flow through the trough to the 

 vessel still nearer the temperature of maximum density. The tem- 

 perature of water at maximum density was thus shown to be 39*1. 



223. Paddle Apparatus, by means of which Dr. Joule determined the 

 dynamical equivalent of heat. Described in ' Philosophical 

 Transactions ' for 1850, p. 65. (Sir William Thomson.). 



592. Drawing of Mercurial Air Pump (1872). (J. P. Joule, D.C.L., 

 F.R.S.) 



By alternately lifting and lowering the bulb attached to the flexible 

 tube, the air being dried by the admission of sulphuric acid through 

 a glass valve at the upper part of the perpendicular tube,- a very 

 excellent vacuum may be obtained in a short time. 



1118. Surface Electro -Mag net made in 1840. When fully excited, 

 the armature is retained with a force of upwards of a ton. 

 (J. P. Joule, D.C.L., F.R.S.) 



1144. Electro-Magnet for Induction and Diamagnetic Experiments, 

 made in 1850, of a broad plate of annealed iron, so as to 

 obtain a large induced power from a small voltaic source. 

 (J. P. Joule, D.C.L., F.R.S.) 



The coil is composed of a bundle of copper wires, and has a 

 resistance about equal to that of a Daniell's cell, exposing a surface 

 of 1 ft. square. 



1190. Drawing of a Dip Circle. (J. P. Joule, F.R.S.) 



The needle, constructed of a thin ribbon of annealed steel, weigh- 

 ing 20 grains, is furnished with an axis made of a wire of standard 

 gold. This axis is supported by threads of the Diadema Spider 

 attached to the arms of a balance suspended by a fine stretched wire. 

 The whole is hung by a wire which can be twisted at the head 

 through 180°. At the bottom is attached a paddle immersed in 

 castor oil, which brings the instrument speedily to rest in a fresh 

 position. The deflections are read off by a short focus telescope, 

 placed on an arm revolving on an axis in the centre of the circle. 

 With this instrument the dip can be determined within the fraction of 

 a minute of a degree in less than a quarter of an hour. 



With this drawing is exhibited a specimen of the thread of the 

 Diadema Spider, also thread of the Diadema Spider cocoon. 



