Tide GAGE, the name of an instrument used for determining the height of the tides by Mr Bayly, in the course of a voyage towards the south pole, &c. in the Resolution and Adventure, in 1772, 1773, 1774, and 1775. This instrument consists of a glass tube, whose internal diameter was seven tenths of an inch, lashed fast to a ten feet fir rod, divided into feet, inches, and quarters: this rod was fastened to a strong post fixed upright and firm in the water. At the lower end of the tube was an exceeding small aperture, through which the water was admitted. In consequence of this construction, the surface of the water in the tube was so little affected by the agitation of the sea, that its height was not altered one tenth of an inch, when the swell of the sea was two feet; and Mr Bayly was certain, that with this instrument he could discern a difference of one tenth of an inch in the height of the tide.