180 LECTURE XX. 



of which it forms the side, it is rendered so friable as to be worked with 

 ease. More commonly, however, the force of gunpowder is employed, and 

 rocks are generally blasted with great convenience by an explosion of this 

 powerful agent. A hole being bored to the depth of three or four feet, the 

 powder is placed at the bottom, and a wire being introduced, small stones 

 and sand are rammed round it, and the wire is withdrawn, leaving a com- 

 munication for firing the powder by means of a train of sufficient length to 

 insure the safety of the workman. It is said that the explosion is more 

 efficacious when the powder does not fill the whole of the cavity ; this, 

 however, appears to require confirmation. The chemical powers which are 

 the ultimate causes of the operation of gunpowder, belong to a department 

 of philosophy which it is not our business to investigate : but the elasticity 

 of the gases and vapours which are extricated, as modified by the heat 

 which accompanies their production, will be considered and explained in 

 the subsequent divisions of this Course of Lectures. 



LECT. XIX. ADDITIONAL AUTHORITIES. 



The subjects embraced in this Lecture and Lecture XVI. are of so miscellaneous 

 a nature, that a detailed list of authorities would be very tedious. We refer for ge- 

 neral information to the Encyclopedic Methodique Arts et Metiers. Machines ap- 

 prouvees par 1' Academic Royale des Sciences, 4to and fol. 1735-89. Bailey's Plates 

 of Machines, approved by the Society of Arts, 2 vols. fol. 1782. Repertory of Arts, 

 1794 Nicholson's Journal, 1797 Philosophical Magazine, 1798 An- 

 nals of Philosophy, 1800 Mechanics' Magazine. Newton's Patents. Engi- 

 neers' and Architects' Journal. The Encyclopaedias Britannica and Metropolitan, 

 &c. &c. 



Treatises. Borgnis, Theorie de la Mecanique Industrielle, 4to, Par. 1821. Du- 

 pin, Introduction d'un Nouveau Cours de Geometric et de Mecanique appliquees 

 aux Arts, 1824. Second Discours, 1825. Geometric et Mecanique appliquees aux 

 Arts, 3 vols. 1825-8. Christian, Traite de Mecanique Industrielle, 3 vols. 4to, 

 1822-5. Hachette, Traite Elementaire des Machines, 4to, 1828. Barlow on Ma- 

 nufactures and Machinery, 4to, 1836. Ure's Dictionary of Arts, Manufactures, 

 and Mines, 1839. Supplement, 1844. 



LECTURE XX. 



ON THE HISTORY OF MECHANICS. 



THE order which we have pursued in considering the various depart- 

 ments of mechanical science, has been in great measure synthetical, dic- 

 tated by the plan of proceeding logically from the most simple principles to 

 their more complicated combinations, so as to build at every step on foun- 

 dations which had been firmly laid before : and this method is unquestion- 

 ably the best adapted for the expeditious progress of a student in sciences 

 with which he is unacquainted. But having once acquired a certain 

 degree of knowledge, he is anxious to be informed by what steps that 

 knowledge was originally obtained, and to what individuals mankind is in- 

 debted for each improvement that has been successively made. Hence, 



