35 



part of an inch in pressure. Thus a mere inspection of the posi- 

 tion of the carriage on the beam, together with the reading on the 

 notched wheel, gives the air pressure to four decimal places. Owing 

 to the fi"ictional resistances and other influences unavoidable in all 

 such mechanisms, the fourth figure of decimals can not be regarded 

 as having a real pressure significance. 



The readings are as accurate, probably, as the best eye readings 

 •of a good mercurial barometer, that is, to about one-thousandth part 

 of an inch. 



The record is not appreciably affected by changes of temperature 

 that affect the whole instrument uniformly. 



72. C OTnpensdted siphon harograph, Marvin system. — This instru- 

 ment is illustrated in Figures 25 and 26 and belongs to that class in 

 which the record is made mechanically without interposition of any 

 clockwork or electric mechanism to overcome friction, etc. To se- 

 cure satisfatory records on a highly magnified scale by this method 

 it is indispensable that the friction involved in writing the magni- 

 fied record be removed to the last degree. Experience has demon- 

 strated that this has been accomplished in the arrangement described, 

 and this instrument proves to be exceedingly accurate and far more 

 reliable than any of the types heretofore employed. The clock and 

 electrical mechanisms required in the older instruments act in a 

 certain sense indirectly and are the cause of some errors. The 

 weakening of batteries or failure of electric mechanisms from time 

 to time also results in interruptions in the record that do not occur 

 in the system of direct mechanical registration now to be described. 



73. Compensated siphon. — The barometer of this instrument is a 

 special form of siphon clearly shown in Figure 25 and with dimen- 

 sions marked in Figure 26. The long and short branches consist 

 of simple, straight tubes. These are narrowed down at the lower 

 ends where they are fitted into the upturned branches of the bend, 

 or lU. The tubes, in fact, form hollow stoppers carefully fitted and 

 ground in. The top of the U above the ground joints are provided 

 with bells, or cups, of ample size, which have a lip formation on 

 one side. This 3-piece construction enables the barometer to be filled 

 in a most satisfactory manner, but more especially the siphon after 

 being once filled can be assembled or dismantled and transported 

 without loss of the vacuum. The mercury in the open leg of the 

 siphon in the course of time becomes more or less fouled with oxida- 

 tion, the accumulation of dust, etc. The construction described per- 

 mits of removing the short branch of the siphon at any time with 

 very little trouble. The tube and excess of mercury can then be 

 thoroughly cleaned and replaced. 



74. Filling and installing the siphon. — The ordinary siphon tube 

 made in one piece of any considerable size is very difficult to fill and 

 secure a good vacuum, and it can not then be easily cleaned or trans- 

 ported. The 3-piece construction already described overcomes these 

 difficulties and requires only that the long, straight branch be 

 carefully filled. This may be done by almost any of the methods 

 already described in paragraphs 33 to 38, but the air-pump method 

 is undoubtedly the best. 



When the siphon is to be installed it will be well to prepare the 

 ground joints by the application of a little lubricant, such as vaseline, 



