14 



THE SUN, STARS, AND PLANETS 



An ingenious scheme of telling time was by means 

 of water. Water was placed in a tank and allowed to 

 flow out through a small hole into a little tank where 

 there was a float made of wood. As the water rose the 

 float rose and indicated the hours by marks on the side 

 of the little tank. Nowadays clocks are used in which 

 wheels are allowed to turn slowly by means of a pen- 

 dulum. See Section 64. Next year we shall study more 

 about clocks. 



Experiment 8. The Sand-glass, 

 Apparatus: Two small bottles, two stop- 

 pers, a piece of glass tubing J/" diameter, 

 small triangular file, fine sand. 



a. Bore a hole in the two stoppers for 

 the glass tube and insert the tube in the 

 stoppers with their tops together. The tube 

 should not extend beyond the stoppers. Fill 

 ono. bottle with fine, dry sand, insert the 

 stoppers into the bottles and the sand-glass 

 is finished. 



b. How long does it take for the sand to 

 run through? Keep changing the amount 

 of sand until it takes exactly one minute, or 

 exactly two minutes for the sand to run 

 through. Is this an hour-glass? A sand- 

 glass made like this and which would run 

 for three and a half or four minutes would 

 be useful for timing the boiling of eggs. 



