Mr. Mageough's Method of mounting Thermometers. 367 



tube has moved, measuring an angle equal to 90° less by the 

 angle cCS. 



If the arch AS and the tube be graduated, the correspond- 

 ing divisions are easily noted as the mercury rises or sinks, 

 and the divisions can be marked on the former at leisure. 



Because in the tubes commonly to be obtained, the distance 

 cS, of the point of suspension above the axis of the tube, must 

 be very small, the adjustment is rendered comparatively easy, 

 by making the axle of a piece of thin steel wire bent in the 

 form of fig. 2, the inner part of the curve being as nearly as 



possible half the circumference of the tube ; the curve being 

 underneath, the axle is fastened to the tube by a thread wound 

 four or five times round both, and they may be secured from 

 slipping by a drop or two of varnish. Now it is evident that 

 by increasing or diminishing the distance between the points 

 of support or bearing hooks, the centre of gravity c will be 

 more or less sunk below the points of suspension S, and the 

 distance c S will be obtained without the trouble of filing. A 

 knife-edge is however undoubtedly preferable to the wire. 



To find the centres of gravity on the tube, it is only neces- 

 sary to sling it in a horizontal position, by means of a thread 

 fastened to it at each end, and put over a pin : then a plumb- 

 line, hung from the same pin, will cross the tube at the centre 

 of gravity, the place of which should be marked on the glass 

 by a file or diamond. The arch VPV may be of pasteboard, 

 wood, or metal, and should be attached to the wires SP. At 

 P is a loop or ring by which the instrument is suspended. 



If the figure of the tube be altered, and the bulb contain 

 spirits of wine in the upper part and mercury in the lower, it 

 is evident the distance between the centres of gravity at any 

 two temperatures, and consequently the power of the instru- 

 ment, will be much increased. The shading in the subjoined 

 figure shows the portion of the bulb and tube to be occupied 

 by the quicksilver when the temperature is low. 



Fig. 3. 



It may be added that thermometers of this construction might 

 perhaps advantageously accompany wheel-barometers in large 

 apartments or halls. I am, Gentlemen, &c. 



Feb. 23, 1828. W. Mageough. 



LIX. 2Vb- 



