POINTS FROM WHICH THE PROPORTIONS ARE STUDIED. 375 



and is used to measure the height ; by turning the gauge B, after having 

 completely withdrawn it, it becomes a compass of thickness to measure 

 the great lengths. As to the compass of thickness properly so called, 

 merely turning it will transform it into a small toise. 



On the other hand, in connection with M. Vignardou, j^rincipal 

 of the department of physics and chemistry at the Alfort school, we 

 have constructed a goniometer by the use of which we can dispense 

 with the spirit-level, an instrument very difficult to use on an irritable 

 horse. 



Suppose, for example, we are measuring (Fig. lo(3) the inclination 

 of the segment, AB, upon the horizon, XY. To do this, it will be 

 sufficient to estimate the 

 angle, BAP, formed by this 

 segment with the vertical 

 line, and deduct 90 degrees 

 from the angle obtained. It 

 Is quite easy to arrive at 

 this result by employing the 

 following instrument, which 

 we intend to designate by 

 the name arthrogoniometer, 

 on account of its use. 



It is composed (Fig. 138) of 

 a compass of precision, made of 

 wood, whose branches are 0.65 m. 

 long, and which at the same 

 time is used as a compass of 

 thickness. 



Upon one of these branches 

 a semicircular piece of copper, 

 very accurately graduated, well 

 centred, and having a radius of 

 0.20 m., is fixed by means of 

 movable screws. The centre of Fic*- 136. 



this circle corresponds to the 



axis of rotation of the branches, an axis upon which a small metallic rod 

 0.10 m. long is screwed perpendicularly, upon which freely slides a hollow 

 cylinder supporting a plumb-line. 



In order to use the arthrogoniometer, we stand the horse in his normal 

 equilibrium, and then place the articulation of the instrument over an articular 

 centre, as, for example, the scapulo-humeral (Fig. 137). The observer stations 

 himself about fifty centimetres from the animal ; he is careful not to touch the 

 latter, and recommends the assistant to hold his hand over the eye of the cor- 

 responding side, so that the horse may not be frightened by the examination 



