584 LECTURE LX. 



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

 he proposed, 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 

 expansive 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 sug- 

 gested, about the same time, by David Gorlaeus, and it was afterwards 

 adopted by Descartes,* as a part of his hypothesis respecting the constitu- 

 tion of matter ; which he imagined 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 con- 

 cerning 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 instru- 

 ment remained, however, much limited, for want of an accurate method of 

 adjusting its scale, and it was not till the close of the century, that Dr. 

 Hooke's discovery, of the permanency of the temperature of boiling 

 water, afforded a correct and convenient limit to the scale on one side, 

 while the melting of snow served for fixing a similar point on the other ; 

 although there would have been no great difficulty in forming a scale suffi- 

 ciently natural, from the proportion of the expansion of the fluid contained 

 in the thermometer to its whole bulk. 



It was about the year 1628, that Dr. Harvey 1* succeeded in demon- 

 strating, by a judicious and conclusive train of experiments, the true 

 course of the circulation of the blood, through the veins and arteries, both 

 in the perfect state of the animal, and during its existence as an embryo. 

 Servetus had explicitly asserted, in his work on the Trinity, as early as 

 the year 1553, that the blood performed, in its passage through the lungs, 

 a complete revolution, beginning and ending in the heart ; and Cisalpinus 

 had even expressed, in 1569, some suspicions that the circulation of the 

 whole body was of a similar nature ; but neither of these authors had 

 advanced any satisfactory proofs in confirmation of his opinions. 



In the middle of the seventeenth century, the barometer was invented by 

 Torricelli ; the variation of the atmospheric pressure was discovered by 

 Descartes ; and Pascal made several experiments on the difference of its 

 magnitude at different places, which tended to illustrate the principles, on 

 which the method of determining heights by barometrical observations is 

 founded. 



What Gesner and Aldrovandus had before done with regard to the 

 animal kingdom, was performed, a century later, for the vegetable world 

 by John and Caspar Bauhin, whose works, as collections of all that was to 

 be found on record respecting ftie distinctions and properties of plants, 



* Princip. PLilos. Part IV. 29. 



f Exercitatio Anatomica de Motu Cordis et Sanguinis, Francof. 1628 : his expe- 

 riments were made in 1616. Consult Cuvier, Le?ons sur 1'Hist. des Sci. Nat. 



