743 



HOT-WALL. 



HOUSE. 



740 



as the temperature, watering, and syringing are concerned, is precisely 

 the same as is recommended for the damp stove. 



What are called Palm-houses, ifusa-houscs, Orchidaceous-houses, &c., 

 are merely damp stoves of different dimensions, for the cultivation of 

 those different subjects. 



The only other hothouse distinct from those already noticed is the 

 forcing-house. The treatment which this requires is essentially different 

 from any which has been described, the object being not merely to grow 

 the plants, or to make them produce flowers, but to obtain fruit, and 

 that too at particular seasons. [FORCING.] 



Hothouse plants are peculiarly liable to the attacks of insects, and 

 unless carefully and constantly attended to, these little depredators do 

 a vast deal of mischief. The most common kinds are the Green-Jly, 

 Thrips, Red-tpider, Brown-scale, and ifealy-bug. The first of these is 

 easily conquered by fumigating the house with tobacco, or syringing 

 the plants with an infusion of the same substance. The best remedy 

 for the thrips and red-spider is to syringe well, and keep the house very 

 moist and warm, as those insects cannot live in excessive moisture ; a 

 little of the flowers of sulphur shaken upon the leaves will also destroy 

 them. The brown-scale and mealy-bug are the worst of all that infest 

 plants in hothouses. Bruised laurel leaves strewed upon the passages 

 and other parts of the house are said to destroy them, but these must 

 be used with great caution, as they may not only kill the insects, but 

 the plants themselves. Various other substances are said to destroy 

 them, but, after all, the best and surest remedy is to wash them off. 

 This is perhaps rather difficult where these little intruders are 

 numerous, but after the plants are once clean, it is an easy matter with 

 a little attention and diligence, to keep them so. 



HOT-WALL. Hot or flued walls are constructed hi cold countries, 

 for the purpose of affording warmth to trees placed against them, so as 

 to counteract the effects of frost in autumn, when the wood and buds 

 are maturing, and in spring, when the blossoms and leaves are unfolding. 

 If hot-walls are used for these purposes only, they are productive of 

 great benefit to the plants which they shelter ; but if, as often happens, 

 in addition to this it is attempted by their aid to advance the ripening 

 of fruit in any considerable degree, hot-walls are of very doubtful 

 service. This uncertainty of success arises from the exposed condition 

 of the surface of the wall, and the consequent liability of the heat to be 

 dispersed as rapidly as it is generated, either in consequence of wet 

 causing evaporation, by which the bricks themselves are cooled, or by 

 cold sweeping winds, which prevent any accumulation of warm air 

 from being formed. In mild weather, a hot-wall with a south aspect 

 will forward vegetation very considerably ; but in proportion to the 

 extraordinary excitement, so will be the check from subsequent 

 chilling blasts which this variable climate is so subject to in the early 

 part of the season. Besides the dissipation of heat on the south side 

 an equal, if not a greater portion, is radiated from the north side of the 

 wall, where it may be said to be entirely lost. It is therefore evident 

 that wherever coals are expensive, hot-walls are not to be recom- 

 mended, except for the sole purpose of rendering a little assistance 

 towards ripening the wood in autumn, and warding off the effects of 

 frosty nights in spring. 



One furnace is allowed for heating about 40 feet of wall, that is, 20 

 feet on each side of the place where the fire is situated. The flues 

 on either side are made to take four courses, or two returns ; the first 

 course being a little above the surface of the ground, and the upper 1 4 

 or 2 feet below the coping. An improvement consists in admitting, by 

 means of a register, a portion of the heat directly from the furnace into 

 the second course of flues. A thick double woollen netting ought to 

 be provided for the protection of the plants on the wall, and so attached 

 to rollers as to be easily rolled up in fine weather, and let down at 

 night. This, with a close-fitting wooden coping, projecting at least 

 12 inches, wUl, in a great measure, obstruct the radiation of heat. By 

 such means very excellent crops of cherries have been obtained at an 

 early period of the season ; and as this species of fruit is precarious to 

 force in a hothouse, a portion of a hot- wall where the trees are planted 

 permanently may be very properly set apart for such an object. 



Instead of flues, hot-water pipes might be introduced into the cavity 

 of a common hollow wall, a little above the level of the border. One 

 range of 4-inch pipe would be sufficient for at least 200 feet of wall, the 

 boiler being placed at the back of the wall, and having an upper and 

 lower pipe extending 100 feet each way. If, as sometimes happens, a 

 glass case is erected in front of a hot-wall, a hot-water pipe should be 

 made to pass along in front of the trees, about two feet from the wall, 

 and the return-pipe only might be placed in the cavity of the wall. 

 A great saving of fuel would be the consequence of such an arrange- 

 ment. 



HOUR, HOUR-CIRCLE, HOUR-LINE. The first word always 

 means the twenty-fourth part of a day, by what revolution soever the 

 cUy Vnay be measured. [TlME.] In angular measure [ANGLE] it 

 signifies the twenty-fourth part of a complete revolution, or 1 . 



Any great circle on the sphere which passes through the two poles 

 is called an hour-circle, because the hour of the day is known when 

 that circle of the kind mentioned is ascertained upon which the sun is 

 for the time being. But the two semicircles into which the poles 

 divide such a circle belong to different hours, and are twelve hours 

 asunder. In fact, it is a emi-circle which is spoken of under the term 

 hour-circle. [SPHERE, DOCTIIINK OF.] 



The hour-lines of a sun-dial are the lines on which the shadow falls at 

 different hours, and are the intersections of the hour-circles with the 

 plane of the sun-diaL In the times preceding the common use of clocks, 

 the science of dialling was of considerable importance, and many works 

 were written on the subject, in which the forms of such instruments 

 were varied without end, and also the methods of constructing them. 

 One work even announces " a new conceit of reflecting the sunue 

 beames upon a diull, contrived on a plane which the direct beames can 

 never shine upon." In the rest of this article we propose to show how 

 to construct auy plane sun-dial, or rather how to calculate the hour-lines 

 for it by means of spherical trigonometry. A maker of sun-dials should 

 form a table for every species of dial which he wishes to construct, 

 once for all. 



Firstly, suppose the dial to be perpendicular to the meridian, which 

 is the case in the horizontal sun-dial and in the vertical south sun-dial. 

 In this case the line drawn through the dial, in a vertical east and west 

 plane, must be horizontal. The style of the sun-dial is of course to point 

 towards one of the poles. Let o be the centre of the globe, and of 

 SEN the circle on which the dial is to be drawn, and let s r x be the 

 meridian, r being the north pole. Then at noon the shadow of the 

 style (a portion of the line o p) is on o s. Let the position of the sun 

 in the afternoon be in the plane of the hour-circle A p B, and let the 

 real solar time be h (hours or minutes), and turn h into degrees, c., 

 at the rate of 15 to an hour of time. Let H be the angle thus 



obtained. Let a be the angle by which the plane of the dial is to dip 

 below the horizontal plane, and / the latitude of the place. Then in 

 the right-angled spherical triangle P N B, the side p N or P c -N o is i a. 

 and the angle BPN ( = SPA) is n. But N u (answering to the angle 

 SOB) is the angle made by o B, the hour-line wanted, with o N, the 

 noon hour-line : let it be called H'. And by the properties of right- 

 angled spherical triangles, 



tan H'=tan H x sin (I a) ; 



from which H' may be calculated for as many values of n as may V>e 

 necessary. If the dial be horizontal (the most common case) we have 

 (a=0) 



tan n' = tan H sin I; 



and if the north side of the dial dip instead of the south, by an angle a, 

 the formula is 



tan H'=tan H sin (1 + a). 



In the case of a vertical south sun-dial, in which the style must point 

 towards the south pole, we have 



tan H'=tan H x cos I. 



Secondly, suppose the dial to be not perpendicular to the meridian. 

 In that case the circle SPN, perpendicular to the dial, is not the 

 meridian, and it must be ascertained what angle it makes with the 

 meridian, and thence, what hour-line is ON. This being found, the 

 dial is constructed in the same manner as before, and the hour-lines 

 follow the same law, with this exception, that the preceding formula 

 does not measure the angles from the twelve o'clock hour-line, but 

 from some other. It is not worth while to pursue 'this case fxirther. 



HOUSE. The buildings erected for the habitation of well-to-do 

 private citizens, and occasionally those intended to shelter animals, or 

 to receive particular descriptions of machinery, are generically known 

 by the name of houses, in contradistinction to palaces and mamsiom (or 

 the habitations of rulers in the land), and to huts, or hovel*, and 

 caverns, in which the very poor or the uncivilised seek protection from 

 the inclemencies of the atmosphere. As the construction of houses 

 must depend upon the local conditions of climate, of the materials 

 used, and of the state of civilisation which prevails at the period of 

 their erection, it follows that the varieties of house construction must 

 be infinite ; and it will, therefore, be advisable to limit the observations 

 to be made on the subject to the practice which has prevailed in the 

 most distinctly marked countries, or periods, of ancient or of modern 

 times, in this branch of architecture. 



It appears, from Sir G. Wilkinson's ' Manners and Customs of thn 

 Ancient Egyptians/ that the houses of that country, and very probably 



