HYDROSTATICS. 



117 



mistake of tho Roman j.ooj.1.- in \..'.t verb, and, indeed, M 

 regard* tho > whilo the \><- 



Hi-mo; ami in thi* 

 . aro in many rospects the purest, 

 are equally faulty. 



J 1 . .s'ario, and, generally speaking, tho termination to, for ebbe, 

 in the third person singular of thin tonne, in uaed in prose M 

 well aa in poetry. F6-ra, for tarri and * ' : <>otical. Tho 



tormimitiouH of the conditional* in l>-ln: and fl>.be-i-o or fb-bo-no 

 are, with regard to tho pronunciation of tho open f, alike in ail 

 verbs. 



'J-J. The termination gb-bo-no, in all cases, is by many preferred 

 to (b.bf-r<>, on account of ita milder sound. It in, liko tho 

 termination i-a-no, more frequently used in poetry or in solemn 

 prose. Tho termination irf-no, for {-a-no as ta-rit-no for sa-r(- 

 o-no is also not inelegant, particularly in poetry. FC-ra-no, for 

 ta-rtb-be-ro, is poetical. 



III. REMARKS ON THB IMPERATIVE MOOD. 



1 . A milder form of the imperative mood is tho future tense, 

 which piirticiilurly is in use when what ia ordered is not im- 

 nediately to be done, but after some other act ; as, por-td-te 

 qud-sta ISt-te-ra dl-la po-sta, p6-i an-de-re"-te dl-la spe-sie-rt-a, e 

 pren-de-r^-te dii-e 6n-ce di CM-na, carry thia letter to the post, 

 then you will go to the apothecary's shop, and take two ounces 

 of bark. 



2. The infinitive with tho particle 7ion before it ia tho negative 

 form of the second person singular in the imperative mood ; as, 

 non. an-ddr vi-a, do not (thou) go away ! non far qud-sto, do not 

 (thou) do that ! non te-md-rc, do not (thou) fear ! This form is 

 strictly limited to the second person of the singular, and in 

 addressing anybody with the second person of the plural ono 

 must say, ?ion. an-dd-te vi-a, non fd-te qu^-sto, non te-me-te, etc. 

 It is, moreover, evident that the infinitive, as the negative form 

 of the imperative mood, can only be used in addressing persons 

 of an inferior position, as children, apprentices, servants, etc. The 

 omission of cW-vi, thon must, explains the use of this form ; as, 

 non (do-vi) is-se-re, do not be, i.e., thou must not be; non (d-vi) 

 a-vd-re, do not have, i.e., thou must not have ; non (dS-vi) 

 a-md-re, do not love, i.e., thou must not love. 



3. Siino, for sieno or siano, and sii, for sia egli or ella, are 

 erroneous. 



IV. REMARKS ON THE SUBJUNCTIVE MOOD. 



1. Some ancients wrote s-ie for sia, thou mayst be. Poets 

 appear to prefer sia to sii, to prevent the jingling of the two i's. 



2. Also pas-sd-to im-per-fit-to con-di-zio-nd-le, because this 

 tense, as I stated before, generally preceded by se (if), goes 

 together with the conditional present. 



3. Fussi, fussi, fusse, fussimo, fuste, fussero, for fossi, fossi, 

 fosse, fossimo, foste, fossero, is often found in old writers, but is 

 now obsolete ; as, fusli and fuste, for fosti and foste. 



4. F 6s-so-no, in familiar language, is also in use for fossero, but 

 the latter is the better form. 



5. The pluperfect subjunctive tense goes together with the 

 conditional past. 



(The Key to Exercise 31 will le given in Hie next lesson.) 



HYDROSTATICS. VI. 



MODES OP DETERMINING SPECIFIC GRAVITY (continued) 

 HYDROMETERS TABLE OF SPECIFIC GRAVITIES. 



IN our last lesson we examined the mode of ascertaining the 

 specific gravity of a liquid, we must now see what is the mode 

 of procedure in the case of solids. Happily we are not limited 

 to one method, there being several from which we can choose 

 that which seems tho most convenient for tho special case in 

 hand. We will begin with the simplest. Suppose we have a 

 powder, or some small fragments of a solid substance insoluble 

 in water, we should use the small flask shown at Fig. 13 in the 

 last lesson. 



Fill the flask with water, taking care to avoid bubbles, insert 

 the stopper, and, by a piece of rag or blotting-paper, adjust the 

 level of the liquid to the mark on the neck ; then carefully wipe 

 off all moisture adhering to the sides. Having put the flask 

 into one scale, put the counterpoise into the other, and add 

 weights till they balance ; you will thus obtain the weight of 



water which the flask bold*. If the amount is already known, 

 this weighing may be dinpeuned with. Next, carefully weigh 

 the subaUnco who** specific gravity you want to ascertain. 

 The stopper must now be removed from the flask, and the 

 powder or small fragments dropped in. They will, of coarse, 

 displace some of the water, which will flow over. The stopper 

 most then bo carefully replaced, the superfluous liquid wiped 

 away, and the flask again weighed. The difference between 

 this weight and the weight of the solid and water together will 

 show how much water is displaced that is, the weight of a 

 quantity of water equal in bulk to the solid. Divide the weight 

 of tho solid by this and we shall have the specific gravity re- 

 quired. Here is an example which will explain it better than a 

 mere description can do. 



Grains. 

 Some plooes of brass wire were taken, the weight of which 



was found to be ........ 101-0 



The bottle filled with water weighed ..... 902-0 



After an equal bulk of water was displaced by the brass, the 

 flask weighed ......... 



Therefore the water displaced weighed 



101 



MO-6 



13-4 



And the specific gravity of the brass is - 8'145 



Brass being a compound metal, composed of copper and zinc, 

 its specific gravity varies slightly according to the proportion 

 in which they aro mixed, the drawing of it into wire also makes 

 a difference, as tho particl :s are forced different distances apart. 



In ascertaining specific gravities, great care has to be taken 

 to avoid air-bubbles. Some substances are full of pores, into 

 which the air penetrates, and bubbles cling to the surface of 

 others, and materially interfere with the accuracy of the results. 

 When great exactness is required, tho substance is dipped into 

 the water when boiling, and 

 thus the air is expelled. It is 

 then set by to cool down to 

 60, and weighed as before. 



Another mode of ascertain' 

 ing the specific gravity of any 

 body is based on the principle 

 of Archimedes, that a body im- 

 mersed in liquid loses a portion 

 of its weight equal to that of 

 an equal bulk of the liquid. 

 It is very simple, but requires 

 care ; a special arrangement of 

 the scales is also a great ad- 

 vantage. 



The body is first weighed in 

 the ordinary way, a fine hair or 

 thread having been fastened round it. It is then by this hair 

 suspended from a hook underneath the scale-pan, and allowed 

 to dip into a vessel of water, and the weight again ascertained. 

 Tho difference between the two is the weight it loses in water 

 that is, the weight of an equal bulk of water, and if we divide- 

 its weight in air by this, we obtain the specific gravity. Some 

 times the weights are left unaltered in B, and others added tc 

 A after the solid is immersed, till the scales again balance ; we 

 have then merely to divide the weight in B by that in A. The 

 result is exactly the same whichever way we adopt. The follow- 

 ing, then, is the general rule : Divide the weight in air by the 

 loss of weight in water, the quotient will be the specific gravity 

 of the substance. 



Grains. 



Thus, a stone weighed in air . . . . 286 



Immersed in water it weighed . . . 174 



Fig. 14. 



Loss of weight in water 



Ita specific jrrarity, therefore, is ??f 



112 



2-553. 



We can, on thia principle, ascertain the specific gravity of a 

 liquid by weighing a substance in it, and then in water. Since 

 it displaces an equal bulk of each, the loss of weight in the 

 liquid divided by the loss in water will give the required specific 

 gravity. 



The two plans we have mentioned are used when the snb- 

 atance is insoluble in water. Many crystals and chemical sub- 

 stances, whose specific gravity it is important to know, are. 



