July 31, 1879] 



NATURE 



32J 



barometric needle ; it describes, consequently, an arc of 

 a circle of the same radius as the point of the register. 

 This second microscope has a cross of threads like the 

 first, but without a movable thread. The first microscope 

 being pointed to an hourly sign, the axis of the second is 

 directed to the corresponding point of the curve. The 

 sine of the angle of position of its lever, multiplied by a 



Fig. 3. — Thermograph. 



constant faclJS'aii^ increased by a number equally con- 

 stant, gives the height of the barometer at the moment 

 selected. When the reading of the curve is made and 

 verified, the sheet of blackened paper is taken off the 

 cylinder and soaked in a weak solution of gum-lac or of 

 copal in alcohol. By this means the smoked sheet is 

 " fixed," and thereafter put in a portfolio. 

 The feeblest barometric variations are thus shown with 



The thermographs act in an analogous way, except that 

 the motor is here formed by a Bourdon tube of hammer- 

 hardened copper, with a very elongated elliptical section, 

 and twisted into a sort of spiral, the thread of the spire vary- 

 ing from 2 to 3 centimetres, according to the purpose for 

 which it is intended. A B (Fig. 3) represents one of these 

 tubes, forming a little more than one spiral. This tube is 

 exactly filled with alcohol, and closed at its two 

 ends. The expansion of the alcohol compels 

 it to untwist; but in order that its elasticity 

 may be preserved, the internal pressure, cor- 

 responding to the highest temperatures to be 

 reached, should not exceed 8 or 9 atmospheres. 

 This pressure is all the greater in proportion as 

 the thread of the spire is short. The twisted 

 tube is fixed by one of its ends, the other free 

 extremity bears the needle which gives the in- 

 dications. At Montsouris this needle is 5 centi- 

 metres long, as is also that of the barometer. 

 The process of reading the curves is thus ex- 

 actly the same for all the needle registers. 

 The new thermograph of Montsouris Observa- 

 tory contains seven needles acting on the same 

 cylinder. The first needle (No. i. Fig. 3) be- 

 longs to the electro-magnet which traces the 

 datum-line, and the hourly signs ; needles 2 

 and 3 belong to the psychrometer ; needles 

 4 and 5 record the temperature of the surface 

 of ground exposed to the air without shelter ; 

 needles 6 and 7 correspond to the actinometer. 

 The psychrometer is formed of two twisted 

 tubes placed outside on the north face of the 

 wooden kiosk, which shelters the cylinder, 

 and perpendicular to that face. Their fur- 

 thest extremity is fixed, the other is prolonged through 

 the wall of the kiosk by a stem of copper carrying the 

 indicating needle. One of the twisted tubes is uncovered, 

 and forms the dry thermometer ; the other is covered with 

 cambric, and kept moist by means of cotton-wicks dipped 

 in small glass cups, connected with a Mariotte flask 

 placed in the kiosk by means of a long and fine tube 

 of caoutchouc. An instrument of this kind has been in 

 use for fifteen months at Montsouris ; its action is re- 

 gular, and its sensibility very great. 



Fig. 4. — Register of the Atmograph. 



great fidelity, and by examination of the curves we readily 

 recognise the influence which the dynamical state of the 

 atmosphere exercises in increasing or diminishing the 

 weight of its pressure on the ground. It should be added 

 that the uniformity of the calibre of the barometric tube 

 annuls almost completely the action of temperature on 

 the indications of the instrument. 



I 



Fig. 5. — Anemo>;i.ti-li. 



The earth-thermometer is composed of two parts : — A 

 thermometric reservoir of black copper is placed on the 

 surface of a mass of vegetable mould, the top of which is 

 flush with the platform of the roof of the kiosk ; this 

 reservoir communicates by a capillary tube of copper with 

 a twisted tube placed in the kiosk under the cylinder of 

 the register. When the temperature of the ground rises, a 

 portion of the alcohol passes from the upper reservoir into 

 the twisted tube, the pressure increases and the tube un- 

 twists ; but the twisted tube itself and its capillary tube of 



