ON TIDES AND TIDAL ACTION IN HARBORS. 215 



stream of water flows slowly from an elevated hogshead through a coil 

 in a large stove, passes down to the bottom of the float-box and up 

 again into another hogshead, from which it is pumped up every day by 

 the observer into the first one. As but a small elevation of temperature 

 is necessary, this arrangement has proved quite suflicient. 



Another arrangement, devised by Mr. Batchelder, of Boston, and 

 called by him an "Arctic tide-gauge," is in use at Boston, and has com- 

 pared well Avith the ordinary float-gange. It consists of a strong iron 

 tube, about four inches in diameter, firmly bolted to a wharf or pile. It 

 is open at the top, ami has at the lower end a niiiple, to which an India- 

 rubber bag is fastened; the length of the tube being sufficient to allow 

 the elastic bag to be always submerged at the lowest stage of the tide. 

 The bag is supported by a suitable shelf or cage, and is filled with 

 glycerine, which is poured in at the top of the tube. When in this con- 

 dition the glycerine rises and falls within the iron tube in proportion to 

 the varying height and pressure of the column of water above the rub- 

 ber bag, the difference in the height of the two columns being in iDro- 

 portion to the difference of the specific gravity of the water and the 

 glycerine. The parts above described insure protection against floating 

 ice_, and prevent congelation within the iron tube. 



A copper tube about three inches in diameter, closed at the bottom 

 and open at the top, is placed within the iron tube, and floats in the 

 glycerine ; if left free, it would rise and fall with the changing level of 

 this. liquid. The length of the central tube is a little greater than the 

 whole range of the tide. 



Near the upper end of the outer tube there are three spiral springs, 

 fixed at the top and united at the bottom by a plate or disk, from which 

 the central copper tube is suspended. From a stem fixed to the central 

 tube or float, and moving with it, a string or chain leads over a single 

 pulley, and gives horizontal motion to the pencil-carriage of the record- 

 ing-apparatus. 



The distance that the central tube is to move vertically is adjusted to 

 agree with the required range of the pencil upon the record-paper by 

 placing within it suitable weights. 



As the glyceiine rises or falls in the annular space between the iron 

 tube and the central float, the spiral spring at the top is more or less 

 extended, the extension being uniform on account of the cylindrical form 

 of the float. 



It is not necessary that the India-rubber bag be inclosed in a perfo- 

 rated box, for the purpose of preventing oscillation, as it is always sub- 

 merged, and the pressure upon it is equal to the weight of the column 

 of water, having its base at the bag, and its summit at the mean level of 

 the surface- waves. 



A tide-gauge, for observations on an open coast, has been devised by 

 Mr. Henry Mitchell, of the Coast Survey. The graduated scale on the 

 float is read from the shore by means of a spy-glass, the top of the tube 

 serving as index-mark. 



