PERSPECTIVE PERSPIRATION. 



487 



for an artist to draw the ellipses with sufficient ac- 

 curacy for most purposes, by drawing through the 

 proper point in the axle, the major and minor axis, 

 and the two isometrical diameters, thus making eight 

 points in the circumference to guide him. 



If in any case it should become necessary to re- 

 present a circle, which does not lie in an isometrical 

 plane, we may observe that the major axis will be 

 the same in whatever plane it lies : and it will be the 

 picture of that diameter, which is the intersection of 

 the circle with the plane parallel to the picture, 

 passing through its centre. And the major axis will 

 bear to the minor axis the proportion of radius to 

 the sine of the inclination of the line of sight to the 

 plane of the circle. We may observe further, that 

 the diameters of the ellipse, which are to the major 

 axis, as V 2 to V 3, when such exist, are isometrical 

 lines. 



And the representation of every other line paral- 

 lel, and equal to any diameter of the circle, may be 

 exhibited by drawing it equal and parallel to the 

 corresponding diameter in the ellipse. If it should 

 be desired to divide the circumference of an ellipse 

 into degrees, or any number of parts representing 

 given divisions of the circle, it may be done by the 

 following method: 



Fig. 3. 



Let an ellipse be drawn, and on its major axis, 

 A G, a circle described, with its circumference di- 

 vided into degrees or parts in any desired proportion 

 at B, C, D, E, F, &c., from which points draw per- 

 pendiculars to the major axis. They will cut the 

 periphery of the ellipse in corresponding points. It 

 would be difficult, however, in this way, to mark, 

 with sufficient accuracy, the degrees which lie near 

 the extremities of the major axis. But the defect 

 may be supplied by transferring those degrees in a 

 similar way from a graduated circle, described on 

 the minor axis. In this manner an isometrical ellipse 

 may be formed into an isometrical circular instru- 

 ment, or an isometrical compass, which may show 

 bearings or measure angles on the picture in the 

 same manner as a real compass or circular instru- 

 ment would do in nature. 



It may be often useful to have a scale to measure 

 distances, not only in the isometrical directions, but 

 in others also. And this may be done by a series ol 

 similar concentric ellipses, as in fig. 1, dividing the 

 isometrical diameters into equal portions. The other 

 diameters will be so divided as to serve for a scale 

 for all lines parallel to them respectively. 



Thus in the isometrical squares exhibited in fig. 28 

 PI. LXV. distances measured on the longer diagonal 

 or its parallels, would be measured by the division! 

 on the major r.xis, those depending on the shorter 

 diagonal by the divisions on the minor axis. 



To describe a cylinder lying in an isometrica 

 direction, the circles at its extremities should be 

 represented by the proper isometrical ellipses, anc 

 two lines touching both should be drawn : and in a 

 similar way, a cone ; or frustum of a cone, may be 

 described. A globe is represented by a circle 



whose radius is the semi-major axis of the ellipse 

 representing a great circle. 



It would not be difficult to devise rules for the 

 representation of many other forms which might 

 occur in objects to be represented. But the above 

 cases are sufficient to include almost every tiling 

 which occurs in the representation of models, of 

 machines, of philosophical instruments, and, indeed, 

 of almost any regular production of art. 



PERSPIRATION. By perspiration from the 

 bodies of beasts and men, we understand, 1, That 

 operation by which certain fluid matters, separated 

 from the blood in the thick network of capillary 

 vessels and cells constituting the skin, are changed 

 into vapour (or into fine effluvia,) and in this form 

 escape at the pores of the skin ; 2. Sometimes, also, 

 the secretion and removal from the body of these 

 matters themselves, by the action of the skin. This 

 effluvium is usually so fine, that we cannot see it 

 with the naked eye, whence we call it the " insen- 

 sible perspiration ;" but it becomes visible, if we 

 hold the hand on cold glass or polished metal ; also, 

 if the perspiration is strong, in a cold temperature, 

 or if, from a still stronger perspiration, this vapour 

 is not dissolved in the air, but collects on the skin 

 in drops forming sweat. This perspiration through 

 the skin has much resemblance to the vapour that 

 escapes from the lungs, to the secretions of the 

 membranes lining the cavities of the body, as the 

 stomach, chest, and abdomen, with whi?h secretions 

 it also appears to stand in connexion. The impor- 

 tance of this function will be evident when we reflect 

 that the surface of a full grown man contains fifteen 

 or sixteen square feet, and therefore the quantity ot 

 matter incessantly perspired must be very great, 

 which is confirmed by the accurate observations of 

 Sanctorius (Venice, 1611,) who spent a great part 

 of his life at the balance. He weighed and kept an 

 account not only of all the food that he consumed, 

 but also of every thing that passed, from him, and 

 thereby proved that a great part not only of the 

 fluid, but also of the solid substances that a man 

 consumes, leaves his body by perspiration. Per- 

 spiration promotes two objects very important for 

 the preservation of the bodily structure. One is the 

 purification of the blood from injurious and super- 

 fluous matters. Besides the adventitious compound 

 matters that pass into the blood from particular 

 kinds of food (for instance, onions, &c.,) the carbon, 

 the hydrogen, and particularly the excess of nitro- 

 gen, are carried off from the blood by perspiration, 

 and changed by caloric into gas and vapour, and 

 thus removed from the body. The substance of the 

 body is, in many diseases, particularly in fevers, 

 converted into aeriform fluids by an evaporation so 

 extraordinarily increased and accelerated, that the 

 strongest man is entirely worn away in a few days, 

 without having lost any thing except through his 

 skin. The other advantage of perspiration is the 

 preservation of a suitable degree of warmth in the 

 body, and the reduction of an immoderate heat. 

 Every living body has its peculiar degree of warmth, 

 which remains for the most part the same, whether 

 the surrounding bodies are more or less warm. The 

 temperature of man is about 92 to 99 Fahrenheit. 

 As much caloric is employed in the process of per- 

 spiration (see Evaporation,} it is an important means 

 of cooling the body, and of conducting off the heat 

 which is incessantly generated within. The greater 

 the heat which the body is exposed to, or the more 

 it is produced within from other causes, as hot drinks 

 and excitement, the greater is the perspiration, and 

 the more actively is the heat conducted off. If the 

 body is exposed to great cold, the operations of the 

 skin are weakened, perspiration proceeds more 



