ThtM, in experiment* where we endeavour to detect or to measure a force 

 by irfrrr-'t the motion which it produces in a movable body, we regard 

 Friction M a disturbing agent, and we arrange the experiment so that the 

 motion to be oberred may be impeded as little as possible by friction. 



2. APPARATUS. 

 Ewrthing which is required in order to make an experiment is called 



A piece of apparatus constructed specially for the performance of experiments 

 is called an Instrument. 



Apparatus may be designed to produce and exhibit a particular phenomenon, 

 to eliminate the effects of disturbing agents, to regulate the physical conditions 

 of the phenomenon, or to measure the magnitude of the phenomenon itself. 



In many experiments, special apparatus is required for all these purposes, 

 but certain pieces of apparatus are used in a great variety of experiments, and 

 there are whole classes of instruments which have certain principles of COH- 

 Ktruction in common. 



Thus, in all instruments in which motion is to be produced there must 

 be a prime mover or driving power, and a train of mechanism to connect the 

 prime mover with the body to be moved ; and in many cases additional apparatus 



fceasary such as a break to destroy the superfluous energy of the prime 

 mover, or a reservoir to store up its energy when not required; and we may 

 liave special apparatus to measure the force transmitted, the velocity produced, 

 or the work done, or to regulate them by automatic governors. 



We may make a somewhat similar classification of the functions of apparatus 

 U-li.nging to other physical sciences such as Electricity, Heat, Light, Sound, &c. 



:*. GENERAL PRINCIPLE OF THE CONSTRUCTION OF APPARATUS. 



There are certain primary requisites, however, which are common to all 

 uments, and which therefore are to be carefully considered in designing or 

 selecting them. The fundamental principle is, that the construction of the 

 iii-trument should be adapted to the use that is to be made of it, and in 

 (articular, that the parts intended to be fixed should not be liable to become 

 displaced; that those which ought to be movable should not stick fast; that 





