ON THE HISTORV OiF TERRESTRfAL PHYSICS. 747 



natuial bodies excite our attention, by their novelty and importance, is Dr. 

 Gilbert, of Colchester: his work on magnetism, published in 1590, contains 

 a copious collection of valuable facts, and ingenious reasonings. He also 

 extended his researches to many other branches of science, and in particular 

 to the subject of electricity. It had been found, in the preceding century, 

 that sulfur, as vvell as amber, was capable of electric excitation, and Gilbert 

 made many further experiments on the natui*e of electric phenomena. The 

 change or variation of the declination of the needle is commonly said to have 

 been discovered by Gellibrand, a professor at Gresham college, in the 

 year \6Q5; but it must have been inferred from Gunter's observations, 

 made in 1622, if not from those of Mair, or of some other person, as early as 

 1612 ; for at this time the declination was considerably less than Burrows had 

 found it in I08O. 



In the beginning of the seventeenth century. Lord Bacon acquired, by hH 

 laudable efforts to explode the incorrect modes of reasoning, which had oc- 

 cupied the schools, the just character of a reformer of philosophy: but his 

 immediate discoveries were neither striking nor numerous. In 1620, he 

 j)roposed, with respect to heat, an opinion which appears to have been at 

 'that time new, inferring, from a variety of considerations, which he has very 

 minutely detailed in his Novum organum, that it consisted in " an expan- 

 sive motion, confined and reflected within a body, so as to become alternate 

 and tremulous; having also a certain tendency to ascend". A similar opinion, 

 respecting the vibratory nature of heat, was also suggested, about the same 

 time, by David Gorlaeus, and it was afterwards adopted by Descartes, as a 

 part of his hypothesis respecting the constitution of matter; which he ima- 

 gined to consist of atoms of different forms, possessing no property besides 

 extension, and to derive all its other qualities from the operation of an 

 ethereal and infinitely elastic fluid, continually revolving in different orders 

 of vortices. -. 



A much more important step, than the proposal of any hypothesis concern- 

 ing the nature of heat, was also made about the year 1620, by Cornelius Drebel, 

 who appears to have been the original inventor of the method of measuring the 

 degrees of heat by a thermometer. The utility of the instrument remained, 

 however, much limited, for want of an accurate method of adjusting its scale. 



