358 THE GRAVITY BUBBLE. 



pins with brass heads; on these there is laid a 

 circular piece of common glass, cut two inches largei 

 than the diameter of the tank. As the ' pins ' are 

 about three-quarters of an inch above the tank, they 

 allow a current of air to pass over the water, and 

 also prevent, to a certain extent, particles of dust 

 from falling in. On the edge of the movable lid 

 I paste some crimson lace, which serves for orna- 

 ment, and also prevents the glass from cutting the 

 hand of any person moving it about. Sometimes 

 I have a circular piece, about four inches in diameter, 

 cut out of the centre of the glass lid, which allows 

 the latter to be lifted off easily. 



A glass syringe to aerate the water occasionally, 

 n camel hair pencil, an ivory crotchet pin, and a 

 pair of gutta percha forceps, complete the whole 

 machinery of the aquarium, the cost of which is so 

 trifling that the poorest person might manage to 

 procure them. 



One great point in favour of an aquarium, and 

 one by no means generally understood is, that having 

 once filled the tank with salt water, it will last 

 for months, and even years, if proper care be taken, 

 without requiring one particle of sea- water to be again 

 added ; for as the water evaporates, the salt falls to 

 the bottom, and the deficiency may be .supplied with 

 fresh water from the cistern or filter. In order to 

 ascertain when the sea-water is of the proper density, 

 you require to have a ' gravity bubble/ which can be 



