664 -HISTORY OF SCIENCE. 



has a relation to life very different from that which she presents in the 

 lofty abstractions of the ancients. The advantage which society and 

 civilization have gained from these applications is a theme which need 

 not here be entered upon. Nor do we propose to adduce a series of 

 examples of such applications, many of which will doubtless suggest 

 themselves to the mind of the reader. In order to illustrate by a single 

 case how science may serve men's ordinary needs, we place before the 

 reader in Plate XVII. a representation of the manner in which the 

 Times newspaper was daily made legible to the representatives of the 

 press in Paris during the close investment of the city by the Germans 

 in i8^2 Photographs of the sheet only a few inches square were 

 taken on paper, and these were rolled up, tied under the wings of 

 carrier-pigeons, and thus conveyed into the beleaguered city. These 

 microscopic photographs were transferred to glass slides as transpa- 

 rencies, and by means of a magic-lantern illuminated by the electric 

 light, the printed characters were projected on a screen whereon the 

 whole newspaper could easily be perused by a number of persons 

 simultaneously. The accumulation of ages of labour and ingenuity, 

 which has made possible the modern arts and sciences concerned in 

 this apparently impracticable feat, will be perhaps more strongly sug- 

 gested to the reader's mind by the contrast of this scene with that 

 depicted on page 3. 



On the speculative side, the main characteristic of the science of 

 the nineteenth century is that it brings into notice the unity and con- 

 tinuity of nature which underlie all the diversities of phenomena. 

 This view is the point to which many lines of research have converged. 

 Some of the more notable steps by which it has been reached have 

 been named in the foregoing pages, as when Galileo perceived in the 

 Jovian system a repetition of our own, when Newton showed that the 

 same force which draws a stone to the earth keeps the planets in their 

 orbits, when Kant and Laplace proposed the nebular hypothesis, when 

 W. Herschel saw that the motions of the immeasurably distant stars 

 are also under the control of gravitation, and when the spectroscope 

 revealed that sun, and stars, and nebulae are made of the materials for 

 the most part identical with those of our earth. Then there are the 

 discoveries and doctrines mentioned in the last chapter which go to 

 support the unity of the organic world the establishment of the iden- 

 tity of electricity, galvanism, and magnetism ; and, lastly, the great 

 scientific generalization of the nineteenth century, namely, the prin- 

 ciple called the Conservation of Energy or Persistence of Force. This 

 principle, which has been already mentioned in Chapter XIX. (page 

 524 et seq.\ asserts that energy is as indestructible as matter; that the 

 quantity of it existing in the universe is invariable, subject neither to 

 increase nor diminution. The various agencies which it is in the pro- 

 vince of physics to investigate Light, Heat, Electricity, Magnetism, 

 Chemical Affinity, Motion- are all so many manifestations of Energy, 



