174 BENJ. PIKE'S, JR., DESCRIPTIVE CATALOGUE. 



wheels, various weights, and a clock, the laws of motion 

 uniformly accelerated or retarded, as well as those of uni- 

 form motion, and that without employing a space for the 

 weights, of more than five and a half or six feet, which 

 causes it to be extremely convenient and demonstrative for 

 a course of lectures. 



Mechanical experiments are of two kinds ; the one relating 

 to the quiescence of bodies, and the other to their motion. 



Among the former are included those which demonstrate, 

 or rather make evident to the senses, the equilibrium of the 

 mechanic powers, and the corresponding proportions of the 

 weights sustained, to the forces which sustain them, the 

 properties of the centre of gravity, the composition and 

 resolution of forces, &c. 



By the latter, or those on motion, are shown the laws of 

 collision, of acceleration, and the various effects of forces 

 which communicate motion to bodies. 



Of mechanical experiments it may be proper to observe 

 to you, that those wherein an equilibrium is formed, will 

 frequently appear coincident with the theory, although con- 

 siderable errors are committed in their construction. This 

 arises from the effects of friction, tenacity, and other causes. 

 The case is different in experiments concerning the motion 

 of bodies ; in which, whatever care be taken to render the 

 proportion of the forces, and the weights moved, such as is 

 required by the theory ; yet the interference of friction, which 

 renders the former apparently more perfect than they really 

 are, causes these to differ from the theory. 



If the experiments are only designed to assist the ima- 

 gination, by substituting sensible objects instead of abstract 

 and ideal quantities, an apparent agreement between the 

 theory and experiment may be sufficient to answer this pur- 

 pose, although it may be produced from an erroneous 

 construction : such experiments cannot, however, impress 

 the mind with that satisfactory conviction that arises from 

 experiments accurately made. 



Dr. Desaguliers tried the effect of falling bodies, by letting 

 a leaden ball fall from the inner cupola of St. Paul's church, 

 whose altitude from the ground is 272 feet. The ball de- 

 scended through this space in four seconds and a half ; in 

 which time, from theory, it should have descended through 

 325 feet, which makes a difference of about one-fifth of the 

 actual descent between the experiment and the theory. Dr. 



