78 SEC. 3. MEASUREMENT. 



giving it a firmer position ; and, moreover, by the axis of the roller being 

 lodged in a horizontal frame, and the dividing circle of the roller as well as 

 the indicating wheel being free at the top, thereby affording much easier and 

 more accurate reading than Amsler's instrument. This arrangement has the 

 advantage that for simple calculation the zero point of the roller can be placed 

 exactly on the zero point of the vernier, when the tracing pencil is at the 

 commencement of the figure. The weight can be separated from the instru- 

 ment, by withdrawing the bolt, and placed in the case by itself. The runner 

 carrying the axis of the polar arm can be moved along the whole length of the 

 quadrangular bar, by which means at every longitudinal scale desired a 

 round number can be obtained for the value of the vernier unit (for example, 

 scale 1 * 500 vernier unit, 2 square meters, or scale 1 1440 vernier unit, 5 square 

 fathoms). The tracing bar is divided into \ mm., and the runner sliding on 

 the same carries on one side a vernier, on the other an index. For adjustment 

 with the index, the most usual or specially desired longitudinal scales are 

 marked with lines on the bar ; by means of the vernier and the divisions on the 

 bar, proportions of measure not previously given can be easily inserted and 

 noted down ; in the same manner, in the case of plans which have been drawn 

 on shrunk paper, the area can be retained in its actual size by a corre- 

 sponding movement of the runner, and the position of the vernier noted down 

 for a certain amount of shrinking. 



318. Planimeter, divided on a glass plate. 



F. W. Breithaupt and Son, Cassel. 



The planimeter consists of a network marked on a glass plate for a certain 

 scale of the meter measure. 



319. Wetli's Planimeter. 



Physiological Institute of the University of Halle (Prof. 

 Bernstein, Director). 



The planimeter is fitted together by placing the six-toothed movement into 

 the centre of the divided disc, whilst the central point of the small glass disc 

 moves at the other end in the screw of the ring encircling the divided disc. 

 Next, the slide with the large glass disc is placed on the three-railed track 

 in such a manner that the horizontal glass disc comes underneath the smaller 

 vertical one ; the latter is then, by means of the screw which is fixed on the 

 ring, regulated in such a manner that it is easily carried along with the hori- 

 zontal disc by friction. 



The pointer moving on the same axis with the divided disc indicates the 

 superficial contents of the figure in square millimeters. The small toothed 

 wheel records every 1,000 square millimeters of the surface. 



IV. MEASUREMENT OF VOLUME. 



STANDARD MEASURES. 



319a. Casts of a Collection of Roman Measures to hold 



liquids. Archaeological Museum, Madrid. 



The originals, of alabaster, are preserved at the Archaological Museum of 



Madrid. They were discovered at the end of the last century in the Torre del 



Mar, Province of Malaga, Spain. 



