48 NEGRETTI AND ZAMBEA, HOLBOEN VIADUCT, B.C., 



point of the needle indicating the minimum temperature. In this thermometer 

 the reading is always from the upper point of the needle, and not from the 

 mercury itself. 



To extricate the needle from the mercury, a magnet is used, when, if the 

 needle is embedded only a few degrees, it can readily be withdrawn without 

 altering the position of the instrument. Should the magnet not be sufficient 

 for the purpose, we simply turn the thermometer from the upright position, 

 slightly elevating the bulb (58*2). The mercury and index will then flow 

 into the small reservoir (58). Should the index not freely leave the tube with 

 the mercury, assist it with a magnet and when the mercury and index are 

 in the upper bulb (58*2), apply a magnet outside, which will attract and hold 

 fast the index ; and whilst thus holding it, again bring the thermometer to the 

 upright position, when the mercury will immediately fall back into the tube, 

 leaving the index attached to the magnet (fig. 4), with which it is guided 

 down to the surface of the mercury, ready for another observation. 



Price, 2 10 



The value of these instruments may be estimated from the following letters, 

 received from gentlemen by whom the thermometers have been tested since 

 their invention. 



LEWISHAM, 1856, February 27. 



" GENTLEMEN, In reply to your note just received, I beg to say that your new Mercurial Minimum Ther- 

 mometer was suspended by the side of two Minimum Thermometers of the best kind of the ordinary con- 

 struction, on the day I received it from you, viz., 1855, November 21, and it has been examined and read 

 every day since, during which interval of time the temperature has varied from 15 to 60. It has acted 

 equally well within this range. In the course of the experiments, it was found that at times differences 

 amounting to 2 and 3 existed in the minimum readings between those of the new mercurial and old 

 spirit thermometers. These differences were found due to two causes. The one occurred at low 

 temperatures, and on reference to independent registers, it was found that the readings of the mercurial 

 were right, the difference being attributable to the sluggishness of the alcohol; and, in the other case, it 

 was found that the index of the ordinary thermometer had unduly moved towards the bulb, the instru - 

 ment having been shaken by the wind. 



"I consider the new Minimum Thermometer a very important addition indeed a more important one 

 than the Maximum Thermometer of your invention, as by its means we can register all observations of 

 temperature by the use of one fluid, and that the recognised standard for the measurement of heat. 



"With respect to your Maximum Thermometer, it acts admirably, and leaves scarcely anything to be 

 desired. It has never been out of order during the four years* I have had it in constant use, and it does 

 not seem possible to put it out of order, except by the destruction of the instrument. 

 " I am, Gentlemen, your obedient Servant, 



"JAMES GLAISHEE, F.E.S. 

 " Messrs. NEGRETTI and ZAMBRA, Opticians." " Secretary to the British Meteorological Society. 



The following is an extract from a letter to the inventors, Messrs. NEGRETTI 

 and ZAMBRA, from E. J. LOWE, Esq., dated Observatonj, Beestrm, near Nottingham : 



"Your Patent Mercurial Thermometer is an admirable invention. I have worked it to my entire 

 satisfaction. I have tested its usefulness in many different ways, every one of which has been perfectly 

 satisfactory. It is certainly a meteorological triumph for which meteorologists must return you thanks." 



Care must be taken not to withdraw the magnet until the index is in 

 contact with the mercury, for, if released before touching, it might plunge too 

 deeply and give a false indication. The rule for re-setting it will be to bring 



* It is now more than twenty -nine years, and still the thermometer is perfect. 



