228 Royal Society .— 



of Mr. L. P. Casella, Optician, called at Kew Observatory with an 

 instrument of the above description, for the purpose of having it 

 compared with the ordinary maximum and minimum thermometers. 

 This comparison proving very satisfactory, and the principle of the 

 instrument commending itseif to Dr. Robinson, Mr. Gassiot, Pro- 

 fessor Walker, and several other scientific men who examined it, 

 Mr. Gassiot requested me to write a short description of it, whioh 

 he thought might be of interest to the Royal Society. For many 

 particulars of this description I am indebted to Mr. Casella and 

 Mr. Hicks, who furnished me with details regarding the construction 

 of the instrument. 



Its chief advantage consists in its furnishing us with a mercurial 

 minimum thermometer, no serviceable instrument of this description 

 having hitherto been made. At the same time it is also capable of 

 being used as a mercurial maximum thermometer. 



The principle of the instrument is briefly as follows : — 



It has a cylindrical bulb nearly 3^ inches long and half an inch in 

 diameter, filled with mercury. This gives a bore nearly ^th of an inch 

 wide, and a scale on which 1° Fahr. corresponds to about ^th of 

 an inch. When the graduation has reached 150° Fahr. or so, both 

 the tube and the scale are made to assume a position at right angles 

 to that which they occupied previously, so that the first portion of 

 the thermometer being vertical, the second will be horizontal. The 

 numbers on the horizontal scale are not, however, hi continuation of 

 those on the vertical ; for in the instrument from which this account 

 is taken, while 150° is the highest division on the vertical scale, the 

 first on the horizontal is —10°, the next 0°, the 3rd 10°, and so on. 

 The reason of this method of graduation will immediately appear. 



Above the mercury there is a small quantity of spirits of wine 

 which extends some distance into the horizontal tube. The quantity 

 of this, and the graduation, correspond in such a manner, that the 

 extreme end of the spirit column denotes the same degree of tem- 

 perature as the mercury. The remainder of the horizontal tube is 

 filled with air. There are two moveable indices in the spirit column, 

 one in the vertical tube, the other in the horizontal, each about half 

 an inch long. The former, B, consists of a fine steel magnet enclosed 

 in glass. This forms the body of the index. At either extremity there 

 is a head of black glass, similar to that which occurs in the index 

 of an ordinary minimum thermometer. A fine hair is tied round 

 the neck of this index, between the body and the upper head ; and 

 it is made to hang down by the side, so that by its elastic pressure 

 against the tube, the index may be kept in its place, notwithstand- 

 ing its verticality. The index in the horizontal tube A, is in all 

 respects similar to that of an ordinary minimum thermometer. 



Let us now suppose the instrument fixed in its position, the first 

 part of the stem being vertical. In order to adjust it, we must first 

 bring the vertical index into contact with the upper extremity of the 

 mercurial column. To do this, let us take two small but strong 

 horseshoe magnets, and lay the one above the other, so that the poles 

 of the one shall overlap to a small extent the corresponding poles of 



