2 Mr. T. Tate on the Construction of 
but to effect this with a sufficient degree of precision, in such a 
position of the globe, would not only involve considerable prac- 
tical difficulties, but would also require the aid of a catheto- 
meter. This constitutes a serious defect in the instrument, for 
the points in this scale should be determined with the greatest 
possible precision. 
In order to avoid these difficulties and sources of error, I have 
constructed a simple instrument of this kind in the following 
manner :— 
The instrument consists of a glass tube, A B, in- 
serted in a light half-pint flask A, the connexion at 
ev being madeair-tight ; a thermometrical scale m 
attached to the tube ; a moveable barometrical scale 
CD; Qa piece of very thin india-rubber tied over 
the top of the tube A B, to keep the external air from 
coming in contact with the strong sulphuric acid 
occupying a portion of the tube and flask. The 
tube AB is about 24 inches long, and about $ths 
of an inch internal diameter ; it is secured to the 
flask in the following manner :—Two perforated 
corks, e and », fitting the neck of the flask, are 
placed on the tube ; the lower one, e, is coated with 
‘a solution of india-rubber to render it impervious to 
air; the upper cork, v, being raised a little, the cork 
e with the tube is pressed down to its proper depth ; 
some pieces of chemical cement, fusible at a gentle 
heat, are now placed above the cork e, and a gentle 
heat is applied until the cement melts ; the upper 
cork v is then brought down to its place upon the melted cement ; 
when the cement has cooled, the tube will be found firmly fixed 
in the flask and perfectly air-tight. Strong sulphuric acid of 
commerce is introduced through the tube by means of a pipette 
the tube being held in an inclined position. ) 
N.B. All this process of cementing may be obviated by having 
a globe and tube bent in the manner shown in the succeeding 
diagram ; but I preferred showing how the instrument may be 
constructed cheaply, and with materials which may be readily 
obtained. 
To form the scale mn, a narrow strip of paper, about 10 inches 
long, may be attached to the middle portion of the tube by means 
of a cement of liquid glue. The scale DC may be formed of a 
thin lath covered with paper, about 10 inches in length; it may 
be simply applied by the hand, or it may be made to slide against 
an upright board placed at the back of the tube. These scales 
are graduated in the following manner :— 
The scale mn must be graduated at a time when the weather 
