NATURAL HISTORY NINETEENTH CENTURY. 651 



the atmosphere cannot be said to date anterior to the period of the 

 invention of the barometer and thermometer. These instruments, 

 as first constructed, have been already described (pages 106 and 168). 

 It is recorded that the height of the Torricellian column was daily 

 observed by Pascal at Paris, by Perrier at Clermont, and by Descartes 

 during the years 1649-50, and the readings were by agreement taken 

 at the same hour of the day " in order 

 to see if anything could be discovered 

 by confronting them with one another." 

 Here we see the synchronous mode of 

 observation which plays so important 

 a part in the modern system of meteor- 

 ology set on foot by Pascal. Boyle 

 (page 230) in 1665 observed the baro- 

 meter in connection with the weather, 

 and he added to its scale the words 

 "fair," "set fair," etc., which are still 

 commonly seen on the wheel-baro- 

 meter. This last form of the instru- 

 ment, however, was the invention of 

 Hooke. The barometer has been con- 

 structed in various forms and modified 

 according to the circumstances for 

 which it is intended. In the marine 

 barometer a part of the tube is made 

 with a very contracted bore, in order 

 to prevent the violent oscillations of 

 the mercury, which the motion of the 

 ship would otherwise cause. One of 

 the best forms for stationary observa- 

 tories is that represented in Fig. 323, 

 and known as Fortiris Barometer. The 

 glass tube is protected by being en- 

 veloped in a copper or brass tube c, in 

 which two slits permit the height of 

 the mercurial columns to be observed. 

 On the edges of those slits the scale is 

 engraved, and there is usually a sliding 

 piece with a vernier (page 212), which 

 enables the observer to read off the height of the mercury with great 

 accuracy. The cistern A D B is cylindrical, the upper part D is of glass, 

 and fits into a box-wood tube E, closed at the bottom with a piece 

 of leather b b. This leather, by the action of the screw shown in the 

 figure, can be raised or lowered in order that when a reading is about to 

 be taken, the mercury in the cistern may be brought to a constant level. 

 It is to this level that the graduations on the scale are referred, and it 



FIG. 323. FORTIN'S .BAROMETER. 



