OF METEOROLOGICAL INSTRUMENTS. 17 



the spirit, viz., to the extreme edge. Then suspend or lay it down as before ; and, as the 

 temperature decreases, the spirit will recede and take the index back with it; but, on an 

 increase of temperature, the spirit will advance, leaving the index to mark whatever extreme 

 of cold may have occurred ; this it does at the end furthest from the bulb, whilst the spirit 

 itself indicates the temperature at the time. If in transit the spirit is separated, it is easily 

 united by a swing or two of the arm, holding the bulb downwards, and when thus united, let 

 the thermometer hang with the bulb down for about ten minutes, to allow the fluid to settle 

 from the sides of the tube. 



Deep Sea Maximum and Minimum Thermometer on Six's principle. For registering 

 past extremes of heat and cold, and showing present temperature. 



This most ingenious and useful thermometer is named after the inventor, Mr. James 

 Six, of Canterbury, and was described by him in the Philosophical Transactions of 1782. 

 Excepting one or two arrangements of metallic thermometers, including a very ingenious 

 instrument by Henry Jo hnson, Esq., T.E.A.S., this is the only thermometer which registers 

 both extremes, in a vertical position. These metallic instruments, however, together with 

 other forms tried, being found wholly inadequate for their purpose, and this alone being 

 selected by the Government, as well as the Royal Society, for registering deep sea 

 temperature, would seem to warrant its description here. As originally made, the Six's 

 thermometer consisted of a long cylindrical bulb, united to a smaller tube of more than twice 

 its length, bent up and down in the form of a syphon, with the cylinder in the 

 centre, and terminated in a smaller oval-shaped bulb at the top. The lower portion of the 

 syphon being filled with mercury, the long bulb, the other part of the tube, and about a third 

 of the small bulb with rectified alcohol ; the remaining part of the small bulb being filled with 

 highly compressed air, which acts as a spring to depress the mercury and cause it to rise in the 

 opposite tube on any contraction (from cold) of the spirit. A steel index enclosed in 

 glass, moves -in each limb of the syphon. The two indices are terminated at top and bottom 

 wi th flattened projecting glass ends, to enable them to move with the least possible friction 

 and prevent the mercury from passing them. They are supported in their position by 

 means of a delicate hair spring. On this principle strictly, but in modified form, the deep sea 

 thermometer has lately been made. Instead, however, of the long centre bulb, a short bulb 

 filled with spirit is joined to the upper end of the syphon, about parallel with but rather lower 

 than the opposite bulb (see form of the tube fig. 210, p. 49), thus keeping the instrument more 

 strong and compact with but one bend, and adapting it better for the comparative rough usage 

 to which it is subjected. The extent to which sea pressure at great depths might 

 effect thermometric indications, however, was not yet known, and therefore the authorities 

 at the Hydrographic Office having applied to the Royal Society on the subject, at their 

 desire, towards the end of 1869, L. CASELLA constructed an hydraulic machine in which 

 to make this interesting test. The result was startling, as, at a pressure equalling 2500 

 fathoms in depth equal 3 tons per square inch, the error equalled 12 to 13 degrees Fahrenheit in 

 excess, whilst in other kinds of registering thermometers, it reached the extraordinary extent 

 of 70 degrees. To remedy this, Dr. W. A. Miller, Vice-President of the Royal Society, 

 suggested an effective remedy, which he thus describes in the ' Proceedings of the Royal 

 Society,' No. 113, 1869 : 



Self-registering Thermometers adapted to Deep Sea Soundings. " Several of these 

 thermometers have been prepared for the purpose with unusual care by Mr. CASELLA, who has 

 determined the conditions of strength in the spring and diameter of tube most favourable to accuracy. He 

 has also himself had an hydraulic press constructed expressly with the view of testing these instruments. By 

 means of this press the experiments hereafter to be described were made. 



"The expedien t adopted (as suggested by Dr. Miller) for protecting the thermometers from the effects of 

 pressure, consisted simply in enclosing the bulb of such a Six's thermometer in a second or outer glass tube, 

 which was fused upon the stem of the instrument in the manner shown in the accompanying figure 48, p. 16. 

 This outer tube was nearly filled with alcohol, leaving a little space to allow of variation in bulk due to expan- 

 sion. The spirit was heated to displace part of the air by means of its vapour, and the outer tube and its 

 contents were sealed hermetically. 



"In this way, variations in external pressure are prevented from affecting the bulb of the thermometer 

 within, whilst changes of temperature in the surrounding medium are speedily transmitted through the thin 

 stratum of interposed alcohol. 



C 



