164 



NEGRETTI AND ZAMBRA, HOLBOKN VIADUCT, E.G., 



324 



FIG. 324. 

 NEGRETT1 AND ZAMBRA'S CELEBRATED 



HORTICULTURAL SELF-REGISTERING THERMOMETER. 



For determining the greatest cold during the night or absence of the observer- 

 This instrument is a Spirit Minimum Thermometer, similar in construction to No 

 53, page 45. The lowest temperature being recorded by a black glass index floating 

 in the spirit. The scale is made of stout zinc, enclosing the tube ; the figures and 

 divisions being boldly marked for quickly and easily reading the indications. 



(fig. 324) Price, 3s. 6d. 



Strongly recommended in all the leading Horticultural Journals as the cheapest and best registering 

 thermometer of the kind for garden purposes. 



Many hundreds of grosses of these registering thermometers have been sold, 

 giving universal satisfaction. Instructions for use given with each instrument. 



SELF-REGISTERING- THERMOMETERS FOR HEAT AND COLD. 



One of the most elegant and ingenious Registering Thermometers is that 

 invented many years back by James Six, Esq., of Canterbury.* It records the 

 highest and lowest temperature (or heat and cold, as it is commonly termed) during 

 any given period of time in an exceedingly simple and convenient manner, and also 

 at any moment showing present temperature. 



325 NEGRETT1 & ZAMBRA'S IMPROVED SIX'S SELF-REGISTERING 

 THERMOMETER FOR HEAT AND COLD. 



Consists of a long cylindrical bulb united to a smaller tube of more than twice its 

 length, bent round each side of it in the form of a syphon, and ter- 

 minating in a small pear-shaped bulb, as shown in the engraving (fig. 

 325). The lower portion of the bent tube is filled with Mercury ; and 

 the long bulb, the upper parts of the tube, and part of the small bulb, 

 with highly-rectified Alcohol. In the tubes will be found two steel needles 

 or indices, terminated at top and bottom with a bead of glass, to enable 

 them to move with the least possible friction. These needles would, 

 from their weight, rest upon the mercury ; but each has a fine hair tied 

 to its upper extremity and bent against the interior of the tube, acting 

 as a spring with sufficient elasticity to keep the index supported in the 

 spirit at any point to which they may be raised in the tube by the 

 mercury. 



The instrument acts as follows : A rise of temperature causes the 

 spirit in the long bulb to expand, and pressing the mercury down the 

 left-hand tube causes it to rise in the opposite one, raising the index 

 with it until the highest temperature is attained. The lower end of the 

 index then indicates upon the engraved scale the Maximum temperature. 

 As the temperature falls, the spirit and the mercury contract, and in 

 returning towards the long bulb the opposite index is carried up by 

 the mercury until the lowest temperature occurs, where it is left 

 indicating upon the scale the Minimum temperature. 



FIG. 325. 



* See " Philosophical Transactions " for the years 1782 and 1790. By some writers the name is spelt 

 Size, and of Colchester. 



