Chap. 7.] fhe Jack lifed by Houfe&reakers. 77 



is, I underftand, commonly employed by die houfe- 

 breakers about the metropolis, to force open doors 

 or windows, or to remove locks or whatever obftruc- 

 tions may be oppofed to them. 



IV. The INCLINED PLANE is that which forms an 

 angle with the plane of the horizon. This angle may 

 be infinitely fmall, and then it is confounded with an 

 horizontal line ; on the contrary, it may be a right 

 angle, and then the plane becomes vertical : between 

 thefe two extremes are comprized all the other de- 

 grees of inclination. 



The principle on which the whole theory of the in- 

 clined plane is founded is this : That the time which a 

 roiling body takes to defcend upon an inclined plane, 

 is to the time in which it would defcend vertically 

 by its abfolute gravity from the higheft part of the 

 plane, in the ratio or proportion which the length of 

 the plane bears to its perpendicular height ; a body 

 therefore placed upon an inclined plane is partly fuf- 

 tained by the plane itfelf, and therefore a weight or 

 power confiderabiy inferior to that of the body is able 

 to fupport it in its fmiation on the plane, or even 

 to caufe it to afcend. On this account it is that in 

 making refervoirs for water, trenches in fortification, 

 or in clearing the earth av/ay for the foundations of 

 buildings, the wheelbarrows f or other vehicles em- 

 ployed are made to afcend upon a plank or fcaffolding, 

 which is placed in the direction of an inclined plane. 



To render this part of the fubjcct perfectly intelli- 

 gible, let A C (fig. 2.) be an inclined plane, then to 

 fuftain the body D upon this plane, and to prevent it 

 from failing, it is nor nece/Tary that the weights d> d y 

 which retain it by means of the cords D e d, fhould be 

 (taken together) equal to the weight of the body D, 



but 



