1867.] 



Observations among the Alps. 



329 



from 40° F. to 45° F. At eleven inclies and a half from the bulb the tube 

 is widened, so that the following inch and a half may represent the range 

 from 45° F. to 115° F. The tube then finishes in a spheroidal chamber, of 

 which the diameters are about an inch and half an inch. The widened 

 portion of the tube may be dispensed with, as the correction which it serves 

 to ascertain may be otherwise found by means of a Table experimentally 

 constructed for each instrument. In that case the spheroidal chamber, in 

 which the tube will then terminate at eleven and a half inches from the 

 bulb, should be made somewhat larger. The fluid employed is alcohol 

 coloured with a drop of pure aniline-blue. A considerable quantity of air 

 is left in the chamber. As a running column has to be read at a particular 

 instant, great plainness is the first requisite for the scale. On this account 

 graduation on the tube has not been adopted ; but at an inch and a half 

 from the bulb is attached an ivory scale, nine-tenths of an inch broad and 

 eleven and a half inches long (or somewhat less if the widened tube be dis- 

 pensed with), its other extremity coinciding with the commencement of the 

 spheroidal chamber. This scale is graduated throughout in millimetres. 

 The number of miUimetres corresponding to each degree Fahr. on the tube 

 of narrow bore, and to every fifth degree from 45° to 115° on the widened 

 tube, should be noted on the back of the scale. 



The principle of the instrument is the same as that of Sir J. Herschel's ; 

 and it is to be worked according to the directions given by him in * The 

 Manual of Scientific Enquiry.' It was devised for mountain use, where the 

 weight of the Herschel and the fragility of its internal thermometer are 

 elements of difficulty. It has also the advantage of being less costly. The 

 air-chamber is made to serve the purpose of the screw in the Herschel, viz. 

 that of altering at will, according to circumstances, the range of the ther- 

 mometer. This is effected by throwing off into the chamber a greater or 

 less quantity of fluid, retaining it there by holding the instrument with the 

 chamber end somewhat lower than the bulb, and working with the remain- 

 ing column. As alcohol expands unequally between its freezing- and boiling- 

 points, a small correction is necessary, depending on the temperature of the 

 alcohol at the time of working. This temperature is ascertained by noting 

 the point in the widened tube, at which the column stands, when the fluid 

 is thrown off into the chamber. The excess of this temperature above 

 45° F., the point from which the fluid is thrown off, has to be added to the 

 temperature between 40° and 45° shown by the head of the working column, 

 in order to have the true temperature. From the openness of the scale, 

 and consequent small range of the instrument for ayiy one adjustment^ it is 

 necessary to select for working a temperature not much removed from that 

 at which the rise in the sun is equal to the fall in the shade. This tempe- 

 rature, which may be called the temperature of equilibrium, will vary prac- 

 tically, according to the solar intensity, from some 5° F. to 20° F. above 

 the temperature of the surrounding influences. By driving the fluid into 

 the chamber until the temperature of equilibrium is represented at a point 



