82 Mr. F. Watkins on the Evolution 



the first time, is characterized by a strong unbroken line. 

 It has obviously two points of minima, and two of maxima; 

 those of minima being nearly in the middle of the two divi- 

 sions of the map, one in each, and the points of maxima co- 

 inciding nearly wath the meridians which separate the divi- 

 sions.. The line of no dip is marked by the broken line. It 

 is taken from Captain Duperrey's map, and rests principally 

 upon observations made by that distinguished officer ni the 

 voyage of circumnavigation of the Coquille, undertaken by 

 order of the French Government for that express object. 

 His observations were at stations in the immediate vicinity 

 of the line of no dip, and occasionally on the line itself. In 

 assigning its position from the former class of observations Cap- 

 tain Duperrey has employed the well-known formula of M. 

 Biot's hypothesis and has computed by it the geographical 

 distance to be allowed for the small observed values of the 

 dip. The facts of observation are thereby in some degree 

 mixed up in their representation with theoretical assump- 

 tions ; but the distances of the places of observation from the 

 line to be determined by them are usually so small, that any 

 error which the theory may have induced must be insensible 

 on the scale on which the present map is drawn. 



The epochs to which the lines of dip and intensity respect- 

 ively correspond are very nearly the same : that of the line 

 of no dip is 1825, and the line of least intensity rests on ob- 

 servations of which the greater part were made between 1817 

 and 1836. 



It will be seen that these lines, which until lately were 

 considered to be identical, and which by many are still sup- 

 posed to be so, with at most merely local interruptions, are 

 systematkallx) distinct, and in one quarter of the globe in par- 

 ticular are separated by a space equivalent to 20 degrees of 

 latitude, or 1200 geographical miles. 



I remain, dear Sir, yours, &c., 

 Tortington, Jan. 1, 1839. Edward Sabine. 



XVI. On the Evohition of Heat hxj Thermo-Electricity. Bt/ 

 Mr. F. Watkins. 



To the Editors of the Philosophical Magazitie and Journal. 



Gentlemen, 



''■"'HE excitement of electricity by the influence of tempera- 

 ■*- ture is evident by the production of the spark, and of the 

 chemical, physiological, magnetical and mechanical effects or- 



