426
[Unitep
magnetism. A freely-suspended magnet always endea-vours to rest in the magnetic meridian.
The variations in the vertical component of the mag-netic dip are the subjects of investigation with the ver-tical force magnet, which is a magnet placed nearly atright angles to the magnetic meridian. It is kept hori-zontal, or nearly so, by weights balanced with extremeaccuracy, and made to vibrate like a balance; and fromits different inclination, the variation of the vertical forceof magnetism is determined.—J. G.
145 Dollond, George, St. Paul’s Churchyard —Inventor.
Atmospheric recorder. This instrument self-registerssimultaneously, on paper, the varying pressures of theatmosphere, the changes of the temperature of air andevaporation, and those of the electrical states of theatmosphere, the fall of rain, the amount of water evapo-rated from a surface of water, and the force and directionof the wind.
[Self-registering instruments, which move equally byclock-work or otherwise, and are made subservient to theregistration of natural phenomena, are of the highestimportance, and particularly so in meteorological investi-gations, where the changes of every element of researchare perpetual, and those which accrue during the nightare of equal importance to those happening during theday.—J. G.]
The atmospheric recorder will correctly register theslightest change which takes place during any period oftime, according to the length of the paper.
The apparatus is composed of a frame of about two feetby three feet six inches, firmly supported upon four pil-lars, the sides of the frame being strongly bolted togetherat two feet from each other. At one-fourth from eachend of the frame, a roller of one foot in circumference isintroduced. To one of these rollers an eight-day clockis attached, which moves it round once in twenty-fourhours. At half right angles above that roller is anotherof the same dimensions, so arranged as to press upon itequally throughout its length. The last-mentioned rolleris for the purpose of keeping the paper in contact with thedriving or clock roller.
The apparatus is represented in the annexed Plate, 95.
The roller at the other end of the frame acts as a restfor carrying the paper to be registered to a platform in themiddle of the frame, which has its face in the same planeas the upper sides of the rollers.
Near the end of the frame, which is placed towards thenorth, is a strong bar, upon which all the fulcrums of theindicators or markers are placed, from which arms of onefoot in length, having spring points at their ends for thebarometer, thermometer, and hygrometer, are struck intothe paper every half hour by a falling lever or frame.For the electrometer, rain, evaporator, force and directionof the wind, ever-pointed pencils are used, which make acontinuous mark upon the paper, with a weight pressingupon them so as to render the marks perfectly distinctwithout interrupting their proper motion.
Beyond the fulcrums there are continuations of thearms of the indicators, to which are applied, by variouscontrivances, the powers which give motion to the indica-tors, in those proportions which are required by the scalesof the eight instruments which mark the various changesof the atmosphere. Each indicator has its properscale placed near to the line of the registering pointsand pencils, so that the last indentures or marks onthe paper may be compared with their respective scales,and the time referred to at which the indication tookplace.
There are also a set of liners which separate each de-partment, and form zeros or boundary lines throughoutthe whole run of the paper, commencing at the point orplace of the indicators, from which any movement orhygrometric change of the paper may be referred to forcorrection.
On each side of the frame, there is a marker for tm 1 >these are governed by a wheel attached to the clock roh®^’which, by a lever and inclined planes, are made to res,ter the time correctly at every half-hour, and sixth homore strongly, for the convenience of counting. I l' e a () fvantage of thus marking the time on both the edges ^the paper is very considerable ; for when the pap® 1 ^taken off, or at any time examined, a line drawn aCI ^ tcorresponding with the opposite marks, will show the coi t 'period at which any change in the atmosphere took p» a 'Having described the general formation of the aPr ^ratns, it will be requisite to give a detailed accountthose parts which are more immediately acted upon by **atmosphere, and the manner in which they are maderegister the results. e
The barometer is upon the siphon principle of abore. Upon the surface of the mercury, in the shot t •leg, is placed a float very accurately counterpoised, leari eonly sufficient weight to compel it to follow the mercu j >and correctly adjusted to that part of the app ara ^ t ,which moves the indicator, when the pressure of the ‘mosphere is at thirty inches. The connexion of the nwith the indicator is so arranged as to give a scale of tb .to one, which has been found to maintain the regist erthe most perfect manner, under comparison with an exlent instrument of the best construction. ..
The thermometrical arrangement consists of ten 111curial thermometers of a peculiar form. These are spended upon an extremely delicate and accurate balaD ^by which a correct register of all the various chang®”this climate have been found to agree with the best tbmometers of the usual construction. They are placedthe north end of the frame, and are screened from the efleof the wind and rain by perforated plates of zinc. j
The hygrometer consists of a slip of mahogany r ^across the grain. This was placed in a cylinder filled wiwater, and suspended from the upper end, with a weigof two pounds at the other end, until it was found Jrepeated examination to be completely saturated, and ^longer to increase in length. The length was then reft* 1to an accurate scale, and the slip of mahogany placed^/the side of the pipe of a stove, under the same suspenand weight, until its shortest length was obtained. ^difference of the two results being carefully taken,scale was formed accordingly. It is placed in a tube, °P ,at both ends for a free passage of air, outside the obs® jjtory. It is suspended and weighted as before, with ^power to act upon the arm of the indicator, quit e ^from the action of the sun or rain, and is found to btremely active and firm in its operation, showing u l ;)0l ^| ]L lopen scale every hundredth of its extremes in drynessmoisture.* , js
The next part of the arrangement to be describe^,the electrometer for thunder-storms and electric c **? n °(orThis is constructed by placing a well-insulated condn , gupon the highest convenient place, from which a Tft -brought down to an insulation on the top of the obs®tory, and from thence to a standard through anothe esulation to a metal disc, between which and a springis a moveable disc attached to a glass or insulating ®for the purpose of connecting it with an accurate suPP j nupon which it can move with the greatest facility• ^connexion with this arm and disc there is a pencil ca I #te dforward to the line of indication. The spring before s
ift fiYP.rl tn n standard at ah/mt tlvron iyw^Vipc ■frrtffl tU .v.
disc
to this a wire is attached and carried into the[® al y 9
vuov , w mio a. itiic aLia^ncu. aiiu callicu. ^y
By this arrangement, the electricity put in motl0 !L c t ofthunder-cloud is received and registered. The ext . y ,this arrangement during a thunderstorm is extremeteresting. When a cloud charged with the electn ^comes within the range of the conductor, the modisc begins slowly to pass from the first disc to the Pdischarging each time a proportion of the electrici y^increasing in rapidity of motion until the discharge ^ ^cloud by lightning takes place. It then falls bac ^