L. CASELLA'S CATALOGUE. 



d 



FIG. 38. 



36. jUthrioscope, the invention of Sir JOHN LESLIE, for ascertaining the absolute 



intensity of terrestrial radiation, with which instrument time is an element ol 

 an observation (fig. 36, p. 13) . . . . 110 



37. FLUCTUATION THEBMOSCOPE, as designed by Dr. B. STEWAET, of the Eoyal 



Observatory, Kew . . . . . . 1 10 



In this arrangement two stems with unequal bores are united to one bulb, and the 

 instrument used horizontally, the scale extending to one hundred divisions. On setting the 

 mercury to the zero, every increase of temperature raises the mercury in the large stem, whilst 

 every reduction in temperature abstracts it from the small one, illustrating a principle in the 

 action of fluids well worthy of extended investigation. 



38. Casella's lercurial Minimum Thermometer, on porcelain scale, with hardwood 



back, and divided on the stem (fig. 38) . . 2 10 



This is the only practical mercurial minimum thermometer hitherto invented, and the 

 result of quite ten years universal effort to achieve. Mercury is the only fluid employed in its 

 make. The bulb and column are of the same size as in the standard maximum thermometers ; 

 and cold is thus registered under precisely the same conditions as heat ; no steel or other index 

 is employed ; whilst the annoyance arising from vaporization, and breakage of the column in 

 the spirit minimum, is entirely avoided. The general form is shown in jig. 38 ; d being a tube 

 with large bore, at the upper end of which a flat glass diaphragm is formed by the abrupt 

 junction of the small chamber a &, the inlet to which at 6 is larger than the bore of the 

 indicating tube. The result of this is, that having set the thermometer, the contracting force of 

 the mercury in cooling withdraws the fluid in the indicating stem only ; whilst on its expanding 

 with heat, the long column does not move, the increased bulk of mercury finding an easier 

 passage through the larger bore into the small pear-shaped chamber attached. It is here most 

 interesting to notice that the weakest natural force is thus capable of resisting the action of the 

 heaviest fluid, as adhesion or capillary attraction seems to be the only force which holds the 

 mercury, and prevents its recession from this point. Great care and a steady situation are 

 essential in using this instrument. 



Directions for Using the Mercurial Minimum Thermometer. Place it in a horizontal 

 position, with the back plate e suspended on a nail, and the lower part supported on a hook /. 

 The bulb end may now be raised or lowered, causing the mercury to flow slowly until the bent 

 part d is full, and the chamber a 6 QUITE EMPTY. At this point the flow of mercury in the 

 long stem of the tube is arrested by adhesion to the diaphragm ft, and indicates the exact 

 temperature of the air at the time. On an increase of heat the mercury will expand into the 

 small chamber a b ; and on a return of cold will cause its recession from this chamber only, until 

 it reaches the diaphragm & to which it adheres. Any further diminution of heat withdraws the 

 mercury down the bore to whatever degree the cold may attain, where it remains until further 

 withdrawn by increased cold, or till reset for future observation. When out of use, or after 

 transit, it may be that raising the bulb may not, at first, cause the mercury to flow from the 

 small chamber as above ; in such a case a slight tap or jerk with the hand on the opposite end 

 with the bulb up, or the application of the extreme tip of the chamber to the flame of a candle, 

 will readily cause it to do so. 



