f>0 NEQEETTI AND ZAMBRA, HOLBORN* VIADUCT, E.G., 



temperature ; and on an increase of temperature the mercury will rise in the 

 tube B, leaving the indicating column in A, registering the extreme degree of 

 cold, or minimum temperature. To re-set the instrument for future obser- 

 vation, simply raise the bulb end of the thermometer until the mercury again 

 comes in contact, and is checked, by the platinum plug. 



This form of Mercurial Minimum Thermometer has one very great 

 advantage over the preceding instrument, viz., it is much less liable to injury 

 or breakage in transit. Price, fig. 69. 220 



69. Actinometer, Sir John HerschelPs (fig. 60), for 

 ascertaining the absolute heating effect of the solar rays, in which 

 time is considered one of the elements of observation. The 

 Actinometer consists of a large cylindrical thermometer bulb, 

 with a scale.considerably lengthened, so that minute changes may 

 be easily seen. The bulb is of transparent glass, filled with a 

 deep blue liquid, which is expanded when the rays of the sun 

 fall direct on the bulb. To take an observation, the Actinometer 

 is placed in the shade for one minute, and read off, itis then 

 exposed for one minute to sunshine, and its indication recorded ; 

 it is finally restored to the shade, and its reading noted. The 

 mean of the two readings in the shade, subtracted from that in 

 the sun, gives the actual amount of expansion of the liquid pro- 

 duced by the sun's rays in one minute of time. For further 

 information see " Report of the Royal Society on Physics and 

 Meteorology." Price, 770 



70. Negretti and Zambra' s improved Isolated Glass 

 Mountings for protecting Thermometer Scales from moist- 

 ure. Many observers having found much trouble in reading the 

 indications of Terrestrial Radiation and exposed Thermometers 

 from the condensation of moisture on the inside of the protect- 

 ing glass tubes or shields, Messrs. Negretti and Zambra have 

 succeeded in perfecting a method of mounting up such instru- 

 ments that quite obviates the difficulty. This improvement con- 

 sists in so melting an external glass cylinder round both ends 

 of the thermometer as to render the shield perfectly air-tight, in 

 fact, to hermetically seal up the instrument in it so that no 

 moisture can possibly accumulate inside the tube, whilst the bulb 



FIG 60 ^ *^ e thermometer is perfectly exposed to the air. 



Negretti and Zambra are now applying this improvement with 

 great advantage to Thermometers, Hygrometers, and many other instruments 

 required for out of door exposure. These isolating mountings will make 

 a slight addition to the cost -of such Thermometers. 



