16 N THE THERMOMETER. 



can be inverted without injury, and the mercury falls into the 

 tube, producing a sound as water does in the water-hammer. 

 When the instrument contains air, the thread of mercury is apt 

 to divide on inversion, or from other circumstances. When this 

 accident occurs, it is best remedied by attaching a string to the 

 upper end of the instrument, and whirling it round the head. 

 The detached little column of mercury generally acquires in this 

 way a centrifugal force, which enables it to pass the air, and 

 rejoin the mercury in the bulb. 



When the glass of the bulb is thin, it is proper to seal the 

 tube as described, and to retain it for a few weeks before mark- 

 ing upon it the fixed points. Thermometers, however carefully 

 graduated at first, are found in a short time to stand above the 

 mark in melting ice, unless this precaution be attended to. Old 

 instruments often err by as much as half a degree, or even a degree 

 and a half in this way. The effect is supposed to arise from the 

 pressure of the atmosphere upon the bulb, which, when not 

 truly spherical, seems to yield slightly, and in a gradual manner. 

 The chance of this defect may be avoided by giving the bulb a 

 certain thickness. Mr. Crichton's thermometers, of which 

 the freezing point has not altered in forty years, were all made 

 unusually thick in the glass. But this thickness has the disad- 

 vantage of diminishing the sensibility of the instrument to the 

 impression of heat. 



We have in this way the expansion marked off on the tube, 

 which takes place between the freezing and boiling points of 

 water. On the thermometer which is used in this country, 

 and called Fahrenheit's, this space is subdivided into ISO equal 

 parts, which are called degrees. This division appears empirical, 

 and different reasons are given why it was originally adopted. 

 Fahrenheit was an instrument-maker in Hamburgh, and as he 

 kept his process for graduating thermometers a secret, we can 

 only form conjectures as to what were the principles that guided 

 him. 



It is more convenient to divide the space between the freezing 

 and boiling of water into 100 equal parts, which was done in 

 the instrument of Celcius, a Swedish philosopher. This di- 

 vision was adopted at a later period in France, under the 

 designation of the centigrade scale, and is now generally used 

 over the continent. The freezing point of water is called 0, 

 or zero, and the boiling point 100. But in our scale, the 



