356 Mr. J. D. Chorlton. Examination of some of the [Mar. 19, 



A new Dip-circle (' Collected Papers,' vol. 1, pp. 575 — 584). 



The instrument was described at a meeting of the Manchester 

 Literary and Philosophical Society (vol. 8, p. 171). 



Joule was not satisfied with the usual method of supporting the 

 needle of a dip-circle in which the cylindrical axle of the needle is 

 made to roll on agate planes, and, after trying several methods of 

 suspension, finally adopted one in which the axle of the needle rolls 

 on silk fibres. His dip-circle received successive improvements, until 

 it finally assumed the form described in the ' Collected Papers,' ou 

 p. 577. A drawing of the instrument was exhibited at South Ken- 

 sington, in the Loan Collection of Scientific Apparatus, 1876, and 

 afterwards remained there. 



J oule says that he could make an observation of the dip with this 

 instrument in about ten minutes, and that the difference between any 

 two consecutive determinations hardly ever exceeded a fraction of a 

 minute. I must confess, however, that I could not work with the 

 instrument to anything approaching that degree of speed and accu- 

 racy. In the first place, the usual method of placing the needle in 

 the plane of the magnetic meridian, i.e., by finding the position in 

 which the dip is 90°, and then turning the instrument 90° in azimuth, 

 is impossible with this instrument, as may be seen from a glance at its 

 form. The only method, then, of finding the magnetic meridian is 

 to take successive observations with the needle in different planes 

 until one is found in which the dip is a minimum ; this is a rather 

 lengthy operation, and may necessitate five or six readings before 

 the actual observations of the dip can be commenced. The chief 

 difficulty, however, in working with the instrument was the great 

 friction between the axle of the needle and the silk fibres ; the original 

 fibres attached by Joule were broken, and I had to attach new ones. 

 In what way the new fibres differed from the old ones I do not 

 know, but in my observations the friction, and consequently the dif- 

 ference, between the readings was so great, and the tendency of the 

 fibre to slip to different parts of the axle, and so to throw the needle 

 out of the vertical plane, together with the vibrations incident upon 

 the method of suspending the vertical circle, so troublesome, as, for 

 all practical purposes, to render the instrument of little use. Joule 

 seems to have sometimes used spider-threads instead of silk fibres, 

 but the only needle I could find belonging to the dip-circle was too 

 heavy for spider- threads. 



Two Air-pumps (' Collected Papers,' vol. 1, pp. 171 — 189, also 



p. 531). 



Of these two pumps the older was used in the research : " On the 

 Changes of Temperature produced by Rarefaction and Condensation 

 of Air." 



