342 Sir W. Snow Harris on a General Law of 



lightning*, wliere it is figured and described. Fig. 1 rcpi-esents 

 this original construction, in which apvn is the glass thermometer 

 ball, about 4 inches in diameter, and p n a fine metallic wire 

 passed air-tight across its centre. This wire is hermetically fixed 

 through the ball by means of short metallic studs, attached to 

 plates of metal cemented in and about two holes drilled on op- 

 posite sides through the glass ; the studs are perforated for the 

 passage of the wire, and are covered by flattened balls of metal 

 screwed air-tight over them, a b is the thermometer tube, having 

 a divided scale attached to it ; its lower extremity, b, is sustained 

 in a reservoir of glass, iv, containing a coloured liquid. The 

 instrument is prepared for experiment by first drawing out a 

 small portion of air from the ball, ap v n, by means of heat, and 

 then immersing the lower extremity, b, of the tube in the coloured 

 fluid ; as the ball cools, the fluid ascends along the scale ; its 

 precise position is regulated by a small valve, v, cemented in a 

 hole drilled through the upper part of the tube, in the way 

 already described. 



When an electrical discharge of a given force is passed through 

 the wire, the fluid descends along the scale and marks the com- 

 parative degree of heat excited in the wire. For the better ad- 

 justment of the fluid to the zero-point of the scale, the latter is 

 so contrived as to be moveable on the thermometer tube. The 

 method of fixing the wire is shown in fig. 2, in which p m and qn 

 are the metaflic plates and balls already described, fig. 1 ; the 

 plates being formed to the curvature of the glass, and firmly 

 cemented to its surface by good sealing-wax. The metallic wire, 

 pn, fig. 1, being first passed through the holes in the brass studs 

 and put gently on the stretch, is secured in place by small plugs 

 of wood, which, pressing the wire against the metal, not only 

 secures it in the hole, but ensures a good metallic contact ; the 

 whole is rendered air-tight by the balls /»?i, which are flattened 

 and screwed upon studs against the plates, from which they pro- 

 ject, a fine washer of leather being interposed : the small valve 

 V is fixed in the same way. The electrical discharge is caused 

 to traverse the wh-e pn by means of metallic connexions inserted 

 into holes drilled in the balls p n. 



5. Although I found this form of the instrument very sensible 

 and efficient as to its operation, yet it was not sufficiently con- 

 venient in practice. I was lience led to bend the thermometer 

 tube so as to place it in a horizontal position, as shown in figs. 3 

 and 4, the extremity, b, of the tube being either bent downward 

 into the vase containing the coloured liquid, as in fig. 3, or other- 

 wise bent upward and expanded into a small ball open to the 



* Letter to Vice-Admiral Sir T. B. Martin, K.C.B., Comptroller of Her 

 Majesty's Navy, &c. Nicol and Co., London, 1823. 



