1170 



METEOROLOGY. 



[KKDUCIXO OBSERVATIONS. 



The cut at JVMJO 1141 girea an idea of the general deduced in thin manner, is a close approximation to that 

 appearance of each kin.1 of cloud just mentioned. obtained by Glauber's hygrouiotrical tables. 



in recording the cl.-uw of clutuU, tho interest is in- 

 creased by also recording the duvction in which tho 

 cloud* are moving, tbo colour of tin' clouds, and tin ir 

 . ity. Tin! height may bo conveniently 

 estimated by supposing G to represent very high clouds, 

 and those floating along the ground ; tho velocity, by 

 supposing 6 to represent those moving at tliu greatest 

 speed, and those motionless. 



To find tho moan daily amount of rain. Whole 

 amount of ruin collected during tho month of December, 

 i an inch. 



31)0 764(0 025 of an inch being tho mean daily 

 62 amount. 



144 

 165 



11 



To find tho moan daily amount of evaporation for 

 December, 1855. Whole amount evaporated, O'CIO 

 inch. 



31)0 -610(0-0197 

 31 



300 

 279 



210 

 217 



7 Therefore, -019_7, or, in oven numbers, 

 0'020 inch ii tho daily amount. 



To find the amount of evaporation in rainy weather. 



Suppose 1"000 inch of water is placed in the evapo- 

 rator, and that a rain-gauge, of the same diameter as 

 the evaporator, and placed at the same level, has recorded 

 0-510 inch of rain since last observation, whilst tho 

 evaporator is found to contain 1-444 inches. Then 



1*444 inches the amount in evaporator. 

 0-510 amount of rain. 



-0-934 difference. 

 1 -000 amount of water placed in the evaporator. 



OC6 of an inch is the amount due to evaporation. 



To estimate the force of wind. 



Anemometers being expensive, the majority of me- 

 teorologists estimate tho wind's force. It is recom- 

 mended that represent a calm and a hurricane, for 

 this estimate is easily converted into 1 Ib. pressure on 

 tho square foot ; the square of tho estimate will ropro- 

 buut tho Ib. pressure 011 the square foot ; viz. 



= Olb. 



1-1 



2=4 



3-9 



4-16 



6-23 



6-36 



By practice, a near approximation to tho truth can bo 

 obtain 



Mr. i:.-lvillc, in lii.i Minnal of the Raromcter, gives 

 tho following concise table of factors for deducing the 

 temperature of tlio dew-point, from tho temperature of 

 the air and that of tho temperature of evaporation : this 

 t*blo originally appeared in the Greenwich Magtieiical 

 and Meteorological Obttnxitioiu for l&M. The dew-point, 



Multiply tho difference between the wet and dry bulb, 

 by tho factor corresponding to tho temperature of the dry 

 bulb, and subtract the product from tho latter, trbion 

 will give tho temperature of the dew-point. Thus, lot 

 T bo the temperature, W wot bulb, / factor, x tho 

 product, and D the dew-point, then 



x = T Wx/ 

 andD = T x 



Example. Temperature of dry bulb 50, and wet 

 bulb 45, 



6045 = 5 

 5 x 2-1 = 10-5 



50 10-5 = 39 '5 tho dew-point. 

 By Mr. Glaisher's tables this dew-point is 39-7. 



RECOMMENDATIONS AND PRECAUTION'S. Preserve, as 

 much as possible, the continuity of the observations. It 

 is desirable not to change tho positions of the diilVrent 

 instruments, nor even to alter the method of rc-u<lhii; and 



'ering. As it is probable, when two perse: 

 ployed iu taking observations, that each will read slightly 

 diffirrent, a series of simultaneous readings should l>o 

 made, in order to find the personal error of tho observers. 

 If, from any cause, the continuity of tho register should 

 be broken, on no account attempt to fill the blank so 

 caused by estimation. Be punctual to the hours de- 

 termined upon for observations, and read off tho instru- 

 ments in the same way every day ; by doing this, it is 

 less likely that an observation of any ono instrument 

 shall bo overlooked. It is convenient to rule and mark 

 off tho form of observation upon a slate, to be transf 

 to the observatory book, after the whole observations 

 have been made. Before calculating the means, it is 

 recommended to examine each column, to ascertain that 

 no evident error of entry has been made ; an inch in the, 

 reading of tho barometer is a common error. Tho 

 maximum reading of the thermometer is also sometimes 

 entered in tho wrong column, being placed in that of tho 

 minimum ; and vice vend. Decimal arithmetic should 

 always bo used. 



Before fixing the barometer, it should bo ascertained 

 that the space above tho mercury is froo from air. 

 Incline tho instrument slightly from its vortical position : 

 if the mercury, in striking tho upper end of tho tube, 

 produce a sharp rap, tho vacuum is perfect ; if the tap 

 bo dull, or not hoard, there will be air above tho mercury, 

 which must bo driven into tho cistern by inverting tlio 

 instrument and then tapping it gently. If t!i 

 do not succeed in producing this sharp report by tap 

 the instrument will require tho aid of the maker. In 

 fixing the barometer, adjust tho tube vertically by the 

 aid of a plumb-lino. In reading tho instrument, place 

 the eye on tho exact level of the top of tho column of 

 nry, so that each side of the index, and tho top of 

 tho column, shall bo in tho same horizontal plane. 



Tho thermometers should bo protected from rain ; a;, I 

 in making a reading, the observer should do it quickly ; 

 and whilst doing so, avoid touching, breathing on, or in 

 any way worming tho thermometer by tho near approach 

 of his person. 



Sir John Hersehol recommends that every meteoro- 

 logist should take an observation every hour throughout 

 the twenty-four, on four stated days ia the year; viz., 

 March 21, June 21, September 21, and December 21, 



