486 SPECIAL METHODS OF DISCOYERT. 



vanoineter, and plates of mica that Forbes discovered the 

 polarisation of heat-raye, both by reflection and refraction, 

 after Berard, Melloni, and Nobili had failed ; but was unable 

 (in the year 1835), by a thermo-pile and galvanometer, to 

 detect any heating effect in the rays of the moon, even when 

 the rays were concentrated 3,000 times. Gauss, in 1836, 

 invented his combination of a bifilar suspended magnet, 

 theodolite, and scale, and employed it as a magnetometer 

 to discover variations of terrestrial magnetism. In 1843, 

 Wheatstone appears to have invented his rheostat (and 

 Jacobi conceived a similar idea), and also his electric 

 balance, by means of which many discoveries in electric 

 conduction resistance were subsequently made. It was by 

 means of those instruments, &c., that Matthiessen, in 

 1858, discovered the electric conduction resistance of nearly 

 all the metals, also of coke, graphite, selenium, phos- 

 phorus, &c. ; and, in 1 859, that of numerous alloys ; and, 

 in 1860, found the effect of metals and metalloids on the 

 conducting power of copper, and that a minute propor- 

 tion of arsenic diminishes it very greatly. Still more 

 recently, many new facts have been found respecting the 

 accumulation and transmission of electricity by means of 

 those beautiful instruments, the reflecting electrometer, 

 reflecting galvanometer, electric replenisher, syphon re- 

 corder, &c., invented by Sir W. Thomson. 



CHAPTEK LIU. 



DISCOVEBY BY INVESTIGATING LIKELY CIRCUMSTANCES. 



THIS method is one of the most successful, and includes a 

 number of more special ones, such as investigating neg- 

 lected truths and hypotheses ; anomalous, peculiar, or 

 unexplained truths ; peculiar facts observed in manufac- 



