PORTABLE BAROMETEll. 5 



fortion of brass tube is attached, which embraces the bavD- 

 inetcr tube, and its lower edge is, in observation, made a tan- 

 gent to the convex surface of the mercury, as in other a\ ell- 

 constructed barometers, and the very narrow slit behind gives 

 abundant light for observation. 



On the bevelled side of the front slit opposite the scale, a Attached anj 

 tliermometer is placed for taking the heat of the instrument, j^Q^jeters • faZ 

 and there is room for the scale of correction placed on nlities in the 

 Uamsden's attached thermometers as well as Fahrenheit's ^^ " ' 



scale, 



A thin brass tube with slits in it turns half round on two 

 pins in the usual manner, and covers the apertures above 

 described in the mahogany tube when the barometer is not 

 in use. The mahogany tube is made rather tapering, and with 

 a ferril at the end opposite the cistern. This fcrril unscrews 

 and shews a steel ring, by which the barorheter may be sus- 

 pended when convenient; and as the mahogany tube is made 

 nearly thirty-eight inches long, there is full space above the 

 top of the barometer tube to put in a thermometer^ which is 

 taken out by unscrewing the ferril, and is to be used as a de- 

 tached thermometer in observation. Along the mahogany- 

 tube is a scale of three feet, carefully divided to inches, t|ie 

 feet being accurately laid down by small dots on the heads of 

 brass pins sunk into the wood, A scale of this kind is always 

 convenient, and may often be of great use. ' 



Having thus described the instrument, a few practical re^ Practical re- 

 marks on the manner of using it may not be superfluous. ^^^ ' 



When I am about to make an observation, about five mi- 

 nutes before I arrive at the place I take out the detached 

 thermometer from its place in the end of the mahogany tube, 

 holding it by the upper end at nearly arm's-length from my Todcicrmin© 

 body and, if the sun shines, in the shade of my person : it very ^^^ tempera- 

 soon takes the temperature of the air, and is not sensibly af- ^^ ^ '^ • ' 

 fected by the heat of the hand. The heat being observed and 

 written down, the barometer is turned up, the brass tube half 

 turned, and the instrument held between the finger and thumb 

 of the left hand above the slide, so as to let it hang freely in a 

 perpendicular position. Few persons, if any, have sufficient 

 steadiness of hand to prevent little vibrations in the mercury 

 in this position.; the hand therefore should be cither rested 



againsr 



