256 A SHORT HISTORY OF SCIENCE 



like a pendulum, a view which, if the valves had any meaning, 

 was now plainly untenable. Harvey therefore set to work to 

 study the beating of the heart and the flowing of the blood, and 

 soon came to the conclusion that there must be a steady flow or 

 streaming in one direction, and not an oscillation back and forth 

 as was generally supposed. But to prove was here, as always, 

 harder than to believe, and much time and labor were required to 

 settle the question. At length, however, by dissections and vivi- 

 sections of the lower animals, and after publishing (in 1628) a bro- 

 chure presenting his facts and meeting objections, Harvey suc- 

 ceeded, with the result that his name justly stands to-day beside 

 those of the Greek and Alexandrian Fathers of Medicine, Hippoc- 

 rates and Galen. It is one of the ironies of fate that while Harvey 

 rightly reasoned from circumstantial evidence that the blood must 

 steadily flow from the arteries to the veins, he himself never actually 

 saw that flowing, a sight which any schoolboy may now see, 

 but impossible before the introduction of the microscope, and first 

 enjoyed by Malpighi in 1661, only four years after Harvey's death. 



In embryology, also, Harvey proved himself an original and 

 penetrating observer. In his day and earlier it was supposed that 

 the embryo, in the hen's egg, for example, exists even at the very 

 outset as a perfect though extremely minute chick, with all its parts 

 complete. This " preformation" theory was opposed by Harvey, 

 whose doctrine of " epigenesis" was substantially that of modern 

 embryology : viz. that the embryo chick is gradually formed by 

 processes of growth and differentiation from comparatively simple 

 and undifferentiated matter, somehow set apart and prepared in 

 the body of the parents. 



ATMOSPHERIC PRESSURE : TORRICELLJ'S BAROMETER. The 

 problem of the existence and nature of voids and vacua had always 

 been an interesting puzzle for philosophers. The Greeks assumed 

 the existence of empty spaces or "voids," and as late as the age of 

 Elizabeth it was the orthodox belief that "nature abhors a 

 vacuum." Galileo, even, held to it in 1638. (Cf. p. 246.) 



Evangelista Torricelli (1608-1647), inspired by the Dialogues of 

 Galileo (1638), published on Motion and other subjects in 1644. 



