118 John Adie, Esq.. on the Marine Telescope, 



plate of glass, which, when used, is to be kept under the sur- 

 face of the water. 



liihiliiliil 



J Foot 



A very convenient size for the instrument represented in the 

 above figure, is to make the length AC, 3 feet, and the mouth A, 

 where the face is applied, of an irregular oval form, that both 

 eyes may see freely into the tube, with an indentation on one 

 side, that the nose may breathe freely, not throwing the mois- 

 ture of the breath into the tube. B is a round plate of glass, 8 

 inches diameter, over which is the rim or edge C ; this rim is 

 best formed of lead, J of an inch thick, and 3 inches deep ; 

 the weight of the lead serves to sink the tube a little into the 

 water. Holes must be provided at the junction of B to C, 

 for the purpose of allowing the air to escape, and bring the 

 water into contact with the glass ; on each side there is a 

 handle for holding the instrument. This size and form is 

 very much that of the instrument brought from Norway by 

 John Mitchell, Esq., Belgian Consul, of Mayville, with the 

 improvement for excluding the breath, and allowing the water 

 to get into contact with the glass, which was not provided 

 for in that instrument. 



The reason why we so seldom see the bottom of the sea, 

 or of a pure lake, where the depth is not beyond the powers 

 of natural vision, is not that the rays of light reflected from 

 the objects at the bottom are so feeble as to be imperceptible 



