MATHEMATICAL SCIENCES SIXTEENTH CENT. 175 



experimental physics since that time. It is still known as the ex- 

 periment of the Magdeburg Hemispheres, and was first performed by 

 Guericke by means of two hollow metallic hemispheres fitted to one 

 another at their edges. When these had been pressed together with 

 a ring of oiled leather, interposed in order to render the junction air- 

 tight, the air was withdrawn by the pump from the interior of the globe, 

 and then the pressure of the atmosphere, no longer opposed with an 

 equivalent force by air within, manifested itself by the resistance it 

 opposed to the separation of the hemispheres. The hemispheres used 

 in Guericke's experiments had a diameter of three-fourths of a Magde- 

 burg ell, and the united force of eight horses was unable to separate 

 them after the air had been pumped out of the interior. The subject 

 of Fig. 77 is a representation of the same experiment with larger hemi- 

 spheres, and is taken from the engraving in the Latin work wherein 

 Guericke announced his discoveries. This book was entitled " Ex- 

 perimenta nova Magdeburgica de vacua spatio" and was published in 

 1672. The illustration shows a number of horses harnessed to the 

 conjoined hemispheres, dragging them in opposite directions, without 

 being able to separate them. 



These experiments excited great interest amongst the scientific men 

 of the time, and especially attracted the attention of Boyle, Hawksbee, 

 Hooke, and Mariotte to the study of the laws which govern the pres- 

 sures of air and gases. It is notable, too, that in the Magdeburg ex- 

 periment we have the first direct demonstration of the great force of 

 the atmospheric pressure, which, in the next century, was utilized by 

 Watt. For in Watt's steam-engine it is the pressure of the atmosphere 

 which is the acting power, steam being employed merely as an expe- 

 dient for removing the atmospheric pressure from one surface of the 

 piston, so that the same pressure, acting on the other side, may mani- 

 fest its effects unopposed. In other words, Watt used steam only to 

 obtain a vacuum within the cylinder of his engine, and derived the 

 immediate motive power of his machine from the same force which held 

 together the Magdeburg hemispheres. 



Enough has been already related to show that with the seventeentli 

 century began that wonderful development of modern physical science 

 which offers so extraordinary a contrast to its unprogressive condition 

 during the preceding ages. We have seen the great changes which the 

 teaching and discoveries of Copernicus and Galileo produced on the 

 notions of mankind concerning the universe, and we have seen how the 

 seventeenth century found science possessed of certain instruments 

 the telescope, the air-pump, the barometer, and the electrical machine 

 each of which became the fertile means of discovery by supplying 

 new and important data. Another instrument of like importance- the 

 invention of which also belongs to this period, remains yet to be noticed. 

 The instrument we allude to is the now well-known Thermometer, but 

 the merit of its invention, or rather the application of the principle on 



