45, CORNHILL, E.G., AND 122, REGENT STREET, W., LOKDON. 65 



the deepest. For whether used in deep or shallow water, unless withdrawn from pressure, 

 its indications would always be more or less in error. Like an ordinary thermometer it is 

 devoid of air, and so quite different from Six's, which, containing compressed air, has a 

 certain internal resistance. Hence it would be more affected by pressure than Six's, how- 

 ever thick the glass of the bulb. By the simple expedient of placing the thermometer 

 entirely in a shield of glass hermetically sealed (see fig. 4 on next page), the effect of external 

 pressure is entirely eliminated. The shield must of course be strong, but need not be 

 exhausted of air. It must, however, render the enclosed thermometer more difficult to be 

 affected by changes of temperature ; in other words, it will make it sluggish. 



" To counteract this sluggishness, in that portion of the shield surrounding the bulb, 

 some mercury is introduced, and confined there by a partition cemented in the shield around 

 the neck of the thermometer bulb. This mercury acts as a carrier of heat from the exterior 

 of the shield to the interior of the thermometer : and the efficacy of this arrangement has 

 been experimentally determined, the instrument thus protected being, in fact, far superior 

 in sensibility to Six's thermometer. 



" So long as the shield withstands the pressure, that is, does not break, the thermometer 

 will be unaffected by pressure, and there is abundant experience to show that such a shield 

 will stand the pressure of the deepest ocean. The greatest pressure can never affect a 

 thermometer so protected. Doubtless the shield will be compressed a little under great 

 pressure, but this can never exert an internal pressure sufficient to have an appreciable 

 effect upon the thermometer. This method of shielding is quite efficacious, and deep-sea 

 thermometers so protected do not require to be tested for pressure in the hydraulic press. 

 The thermometer will simply require to be tested for sensitiveness and for errors of gradu- 

 ation very accurately ; because it is a standard instrument adapted to determine very small 

 differences of temperature as well as large ones, even one or two- tenths of a degree in 

 shallow waters. The test for sensitiveness should determine how many seconds the instru- 

 ment requires to take up a change of 5 deg. rise or fall ; and the time has been found from 

 5 to 10 seconds. 



" A considerable number of these instruments have already been tested at the Kew 

 Observatory with perfectly satisfactory results, which place beyond doubt their value as 

 Standard Deep-sea Thermometers. 



" This instrument possesses great advantages. It has no attached scale, th'e figuring 

 and graduations are distinctly marked on the stem itself, and the shield effectually 

 preserves them from obliteration by sea-water. The part of the stem which forms the 

 background to the graduations is enamelled white,* to give distinctness to the mercury. 



" The hole at the top of the frame is for the purpose of lowering and keeping the 

 thermometer upright until it has reached the water. This is effected by putting a cord 

 through the hole, and both ends of it kept in the hand until the thermometer has reached the 

 water, then one end is let go and the cord pulled on board ; this operation is not imperative, 

 but it saves the thermometer from being knocked about previous to reaching the water." 

 Negretti and Zambia's Patent Standard Marine Thermometer (fig. 72) . Price 2 10 



* Enamelling the bac/c of Thermometer Tubes is on important invention by Negretti and Zambra. Most of the 

 extremely sensitive Thermometers that are now made (such as Clinical Thermometers) would have been almost 

 useless but for this improvement. 



1 cubic foot of Sea Water weighs 62*425 lbs.=557 cwt , or 0-028 of a ton. 



Sea Water freezes at 28. Boils at 213 2 ; variable with the density. 



80 miles from the Island of St. Thomas, at a depth of 3 875 fathoms, the temperature of 

 the Sea was found to be 34 Fahrenheit ; the pressure at this depth 4* tons to square inch. 



The pressure at 100 feet is found by Divers difficult to bear for any lengthened period. 

 Man cannot sustain a greater pressure than 6 Fathoms, or 120 feet. Weights of 6 or 8 

 hundredweight require two Hours to fall through 3 miles of Sea Water, owing to the 

 friction of the water on the rope or wire. Dr. CARPENTER. 



F 



