Professor Draper's Description of the Tithonometer. 409 



point to the top of the siphon c, the distance is three inches 

 and a half. Through the glass at z, three quarters of an 

 inch from c, a third platina wire is passed ; this wire termi- 

 nates in the little mercury cup r, and x and y in the cups p 

 and q respectively. . 



Things being thus arranged, the instrument is filled with 

 its fluid prepared, as will presently be described ; and as the 

 legs ab,b c are not parallel to each other, but include an an- 

 gle of a few degrees, in the same way that Ure's eudiometer 

 is arranged, there is no difficulty in transferring the liquid to 

 the sealed leg. Enough is admitted to fill the sealed leg and 

 the open one partially, leaving an empty space to the top of 

 the tube at c of two and three quarter inches. 



A stout tube, six inches long and one-tenth of an inch in- 

 terior diameter, ef is now fused on at c. Its lower end opens 

 into the main siphon tube ; its upper end is turned over at 

 f and is narrowed to a fine termination, so as barely to admit 

 a pin, but is not closed. This serves to keep out dust, and in 

 case of a little acid passing out, it does not flow over the scale 

 and deface the divisions. At the back of this tube a scale is 

 placed, divided into tenths of an inch, being numbered from 

 above downwards. Fifty of these divisions are as many as 

 will be required. Fig. 2 shows the termination of the narrow 

 tube bent over the scale. 



From a point one- fourth of an inch above the stage d, down- 

 wards beyond the bend, and to within half an inch of the wire 

 z, the whole tube is carefully painted with India ink so as to 

 allow no light to pass ; but all the space from a fourth of an 

 inch above the stage d to the top of the tube a, is kept as clear 

 and transparent as possible. This portion constitutes the sen- 

 tient part of the instrument. A light metallic or pasteboard 

 cap, A D, fig. 3, closed at the top and open at the bottom, 

 three inches long and six-tenths of an inch in diameter, black- 

 ened on its interior, may be dropped over this sentient tube; 

 it being the office of the stage d to receive the lower end of the 

 cap when it is dropped on the tube so as to shut out the light. 



The foot of the instrument k lis of brass, it screws into the 

 hemispherical block m, which may be made of hard wood or 

 ivory; in this three holes, p qr, are made to serve as mercury 

 cups ; they should be deep and of small diameter, that the me- 

 tal may not flow out when it inclines for the purpose of trans- 

 ferring. A brass cylindrical cover L M, L M may be put 

 over the whole; when it is desirable to preserve it in total 

 darkness, it should be blackened without. 



Secondly, of the Fluid Part. — The fluid from which the mix- 

 ture of chlorine and hydrogen is evolved, and by which it is 



