TRANSACTIONS OF THE SECTIONS. 59 



to the construction of the instninient now described, the indications of which are 

 regulated by the lateral motion of compensation bars, composed of thin bars of two 

 metals riveted together that expand and contract in different ratios with change of 

 temperature. Upon one end of a narrow plate of metal about a foot in length (a) are 

 fixed three scales of temperature (h) ranging from 25° to 100° Fahrenheit. 



Upon one of these scales, as shown in the drawing on an enlarged scale, the pre- 

 sent temperature is indicated by the point of a needle { E ), which turns upon a 

 pivot in its centre, and on the other scales register indices (ff, f) are pushed by a 

 pin on the needle (e) to the maximimi and minimimi temperatures, where they are 

 retained by stiff" friction. 



To the needle are attached at equal distances from the centre, by connecting 

 pieces (d d), the free ends of two compensation bars (b b). the other ends of the bars 

 being attached by the plate (c) to the above-mentioned plate (a). 



The motion of the needle is regulated by the lateral motion of these bars with 

 change of temperature. In order to avoid disturbance of indication by lateral con- 

 cussion, two bars are used in lieu of one bar only. 



The compensation bars are composed of brass and steel, in the proportion of two- 

 thirds of brass (which is the more dilatable metal) and one-third of steel, and have 

 sufficient lateral motion to admit of legible scales of temperature, and also sufficient 

 power to overcome the stiff fi-iction of the indices. 



The specific gi-avity of brass being 8-39, and that of steel 7 'SI, it is obvious that 

 no pressure of water can have any effect upon the motive power of the bars, or 

 upon the indications of temperatiire, as imder hydraulic pressure equal to that of a 

 depth of 6000 fathoms of water it acquires a density of 1'06 only. 



The compensation bars are strongly tinned as a protection against sea- water, and 

 the pivots on which the needle and indices move are stronglv gilt. 



In surveying expeditious tliis instriiment may be servicea\»le in giving notice of 

 a variation of depth of water, and of the necessity for talcing soundings. 



A diminution of temperature of water has been observed by scientific voyagers to 

 accompany a diminution of depth, as on approaching hidden rocks or shoals, or 

 nearing land, and also on approaching icebergs. 



The instrument has been suspended by Mr. Glaisher on a thennometer-stand 

 for a period of six months, and read daily in connexion with standard meteorolo- 

 gical instniments, and during this time its readings were approximate to those of 

 the best instnmaents. 



The case of the instrument has been improved at the suggestion of Admiral 

 FitzRoy, and now presents to the water a smooth cylindrical surface with rounded 

 ends, and without any projecting fastenings. 



On a Deep-Sea Pressure- Gauge invented hy Henry Johnson, Esq. 

 By James Glaisher, F.R.S. 



In deep soimdings the pressure of water is too great to admit of measurement by 

 a highly elastic fluid in a small portable instrument. A slight degree of elasticity 

 has been discovered in water itself, and which admits of a vessel of water being 

 used as a measure of the amount of pressure at great depths. 



Mr. Canton, whose experiments were communicated to the Royal Society on 

 Dec. 16, 1762, found, in water subjected to the pressure of an additional atmosphere, 

 a diminution in vohune of one part in 21,740 ; and in water placed under a receiver 

 he found an increase of one part in 21,740 when the air was exhausted. 



Mr. Perkins found a diminution of yf gth parts in the volume of water subjected 

 to a pressure equal to 1120 atmospheres, or about one part in 19,000 for one atmo- 

 sphere. 



A pressure-gauge of metal was exhibited at the Meeting of the British Associa- 

 tion m 1860, and is described at page 203, consisting of a cylinder filled with water, 

 with a solid piston or ram, with a graduated scale, and an index to mark the length 

 of piston forced into the cylinder, compressing the water in it by the gi-eater den- 

 sity of the surrounding water. 



As, however, it is foimd that air- bubbles adhere to the inner surface of the metal 

 cylinder, and the exclusion of air is miportant, a pressiti'e-gauge is now exhibited 

 composed entirely of glass, which is not liable to this disadvantage. 



It consists of a cylindrical glass vessel with a finely graduated long stem or 



