Kr eider — Experiments in Elementary Physics. 333 



diameter — about 8 mm — and rather thick-walled for purposes 

 of magnification. A stopcock is sealed to the upper end, 

 which with good stopcock grease is easily made perfectly tight. 

 The lower end is adapted to receive a heavy walled com- 

 pression tube of rubber. The glass tube is mounted on a 

 block of wood which is painted a good dead-white with hori- 

 zontal dead-black lines about 2 mm wide and one inch on centers. 

 These lines were numbered as shown. In attaching the volume 

 tube to the block thus painted, allowance was of course made for 

 the constriction at the stopcock, so that the graduations should 

 represent equal volumes. The reservoir was laid into another 

 block of wood which was also painted a dead-white. Both of 

 these blocks were provided with tongues which matched verti- 

 cal grooves in the standard, serving as guides. Endless cords, 

 carrying iron counterpoises which hang back of the standard 

 and are, therefore, not shown, supported both blocks with 

 their mountings. This arrangement permits of easy and 

 prompt adjustment of pressures greater or less than the atmos- 

 phere and, providing the cords run in smoothly rounded 

 grooves at the top, rather than in pulleys as they do at the 

 bottom, sufficient friction is obtained to hold the reservoir and 

 tube in any desired relative position without the necessity of 

 clamps. A pinchcock on the rubber connecting tube, so 

 adjusted as to damp the oscillations of the mercury, facilitates 

 the adjustments. 



T is a white, endless tape, one inch wide, on which an inch 

 scale was painted. It was found convenient to extend this 

 scale in both directions from zero, as shown. The tape runs 

 over wooden pulleys at the top and bottom of the standard, by 

 which it is also held flush with the faces of the blocks on which 

 the volume tube and reservoir are mounted. 



One of the chief difficulties in this and similar apparatus is 

 to secure scales sufficiently distinct to be read at a distance. 

 The size of the figures, within obvious limits, is of relatively 

 less importance than the provision of ample space between the 

 figures and between parts of the figures. Where the figures 

 are necessarily limited in size, a very satisfactory scale, legible 

 to the normal eye at a distance of forty feet, can be made of 

 figures about 2 cm in height and 3 mm width of stroke in dead- 

 black on a dead-white surface ; providing at least the same 

 width, 3 mm , of white space is retained between all parts of the 

 figures as well as between the separate figures. 



To illustrate the convenience and the clearness of the demon- 

 stration by this apparatus, suppose the level of the mercury in 

 both limbs to have been brought to 4 on the scale of the vol- 

 ume tube before closing the stopcock. The barometer then 



