SELF-REGISTERING THERMOMETERS. 7 



In this case, however, the above-named difficulty from vaporiza- 

 tion talfes place more readily than when closed with air. These 

 derangements of the spirit-thermometer are readily rectified, and 

 only require occasional, examination to detect them. 



Both the maximum and minimum thermometers may be ad- 

 justed without the magnet, by raising one end sufficiently to 

 allow the index to slide down by its own weight.* 



The ordinary maximum thermometer (Rutherford's) not work- 

 ing well, even in the hands of many careful observers, has occa- 

 sioned several attempts to make one without an index. 



Mr. Green has lately contrived one. The object is effected 

 by inclosing in the bulb a glass valve, which is floated by the 

 mercury to the juncture of the bulb and tube. On an increase 

 of heat the mercury from the bulb passes this valve, but on con- 

 traction from a decreasing temperature, the portion in the column 

 is obstructed, and remains stationary, indicating the maximum 

 point attained. 



To set the instrument for another observation, it is held bulb 

 downwards, and with a gentle jerk the mercury falls and joins 

 that in the bulb ; it is then placed horizontal in the usual way. 



A movable valve-piece is introduced rather than a fixed ob- 

 struction or stricture, as in a new and ingenious maximum ther- 

 mometer by Messrs. Negretti and Zambra, of London, in expec- 

 tation that the observer will find greater ease and satisfaction in 

 readjusting the instrument for observation."}" 



Professor Phillips, of England, has also devised one. His 

 plan is to cut off a portion of the column of mercury by an inter- 

 vening small bubble of air. An increase of heat drives this de- 

 tached portion forward, and leaves it there on a decrease of heat. 



This form is also made by Mr. Green, and possesses some ad- 

 vantages peculiar to it ; but, until experience decide otherwise, 

 we doubt if it can be put in order after accidental derangement, 

 by every observer. The former plans are not open to this objec- 

 tion.] 



* The index of the spirit-thermometer is frequently made of a small 

 cylinder of enamel, which cannot be moved by the magnet. 



t These thermometers being new in plan, particular instructions in 

 regard to suspending and setting them will be given with each instru- 

 ment by the maker, Mr. James Green, New York. 



