24 



REPORT 1866. 



effect exceedingly beautiful. The observer sees au enlarged square picti^e 

 stereoscopic relief, without being conscious by what means it is prodirce 



re m full 

 reoscopic relief, without being conscious by what means it is prodifeed, and 

 without even noticing the apparatus which is employed ; for the whole may be 

 hidden by a screen, having in the middle of its siu-face a square opening upon 

 which the eyes are directed. 



Experiments off Ventnor luith Mr. Johnson's Deep-sea Pressure-gauge. 

 By J. Glaishek, F.R.S., ^r. 



In the year 1861 1 brought under the notice of Section A a deep-sea thermometer 

 and a deep-sea pressure-gauge, both instruments invented by Henry Johnson, Esq. 



Experimental trials were made with these instruments by Sir F. Leopold McClin- 

 tock during his sounding voyage in H.M.S. ' Bull-dog,' on the proposed route of the 

 North Atlantic Telegraph in the year 1860. 



In the experiments made with the deep-sea thermometer, the indications in 

 various deep soundings, to the depth of 1400 fathoms, approximated to those of the 

 best merciu'ial thermometers specially an-anged for temperatures at great depths : 

 my own observations upon one of these instruments daily recorded for six mouths, 

 during which time it was suspended on a stand with other thermometer.^, at the 

 Hoyal Obsei-vatory, Greenwich, also were in agreement. I have taken great interest 

 in this thermometer, which is not liable to be affected by pressiu-e of water ; and 

 which was contrived in consequence of some experiments made by me upon the 

 temperature of the Thames water at Greenwich being frustrated by the pressure 

 of water on the bulbs of the thermometers then used, at the depth of only 24 feet, 

 when the pressure of water would be rather more than two-thirds of the weight 

 of the atmosphere, as represented by the column of water of 32 to 34 feet in the 

 water-barometer. 



The deep-sea thermometer is composed of brass and steel, and the specific 

 gravity of these metals being 8-39 and 7-81 respectively, they are not liable to 

 compression by water, which acquires a specific gravity or density of 1'06 only, 

 under a pressure of 1120 atmospheres, as de- 

 termined by Mr. Perkins. This pressure is 

 equal to a depth in roimd numbers of 6000 

 fathoms. 



In the construction of the instrument ad- 

 vantage has been taken of the well-known 

 difference in the ratios of expansion and con- 

 traction of tlie two metals by variation of 

 heat to form compound bars consisting of a 

 thin plate of each metal rivetted together. 

 These bars assume a slight curve in one direc- 

 tion when heat has expanded the brass more 

 than the steel, and a slight curve in the con- 

 trary direction when cold has contracted the 

 brass more than the steel. 



The indications of the instrimient record 

 the motions under changes of temperature of 

 such compound bars, in which the proportion 

 of brass, the more expansible metal, is about 

 two thirds, and of steel one third. 



To the lower end of a narrow plate of 

 metal, about a foot long, are firmly fixed the 

 ends of two such compound bars, the other 

 ends being free to move to one side or the 

 otlier, according to the action of temperature 

 upon the more dilatable metal. 



The free ends of these bars are connected 

 by lateral arms to a needle moving on a pivot 

 in its centre, at points equally distant from the centre, and the motion of the needle 

 on its axis is regulated by the motion of the bars under the action of the temperatm-e. 



