Registering and Printing Barometer. 21 



without the necessity of any subsequent reductions. The 

 curve for temperature and rain can be recorded on the 

 same cylinder. One battery will be sufficient for all the 

 instruments. 



For the registration of the force and direction of the 

 wind, the printing portion of the mechanism will be of 

 great value ; since it will be an easy matter to connect it 

 with an Anemometer, so as to print the direction in de- 

 grees of the circumference, and the force or velocity in 

 pounds or miles. Whether this method can be applied suc- 

 cessfully to magnetical instruments, remains to be deter- 

 mined; but without having made any experiments in this 

 direction, we see no serious obstacle in the way of its ap- 

 plication. 



The error of reading a first class standard barometer is 

 considerable ; chiefly owing to the difficulty in bringing the 

 surface of the mercury in the cistern to the zero of height. 

 We speak now of that class of barometers where the surface 

 of the mercury in the cistern is brought in contact with 

 the point of an ivory pin. The uncertainty of making an 

 exact contact may effect the readings to the extent of 0.008 

 of an inch. The mean error for reading our standard 

 barometer, Fastre, tube of one half inch diameter, and cis- 

 tern of two inches diameter, is nearly 0.004 of an inch. We 

 also find that there is a personal equation existing between 

 the readings of two observers, mostly due to this cause. 

 This personality, between my assistant Mr. McClure and 

 myself, amounts in the maximum to 0.005 of an inch. It 

 is also found that readings made by the light of a lamp 

 are not the same as the daylight readings ; the difference 

 in some cases amounting to 0.010 of an inch. 



It is proposed to obviate a large source of these difficul- 

 ties by using electrical contact to bring the surface of the 

 mercury in the cistern to the zero. This can readily be 

 done at fixed stations, where meteorological observations 



