ASTRONOMICAL INSTRUMENTS. 

 Fig. 127. 



117 



thence called spring tides, and least at the first and last 

 quadratures, and are thence called neap tides, and the highest 

 tides are near the time of the equinoxes. Price, $4.00. 



Centrifugal Hoops for showing the Earth an Oblate Sphe- 

 roid. (Fig. 128, next page.) An astronomical instrument, 

 to show that the earth, if revolving at all, must revolve 

 upon its shortest axis, and that owing to centrifugal force. 



