a peculiar property of oils, and liquid insulation in general, when subjected to rapidly changing electric stresses, is to disperse any gaseous bubbles which may be present, and diffuse them through its mass, generally long before any injurious break can occur. this feature may be easily observed with an ordinary induction coil by taking the primary out, plugging up the end of the tube upon which the secondary is wound, and filling it with some fairly transparent insulator, such as paraffine oil. a primary of a diameter something like six millimetres smaller than the inside of the tube may be inserted in the oil. when the coil is set to work one may see, looking from the top through the oil, many luminous points--air bubbles which are caught by inserting the primary, and which are rendered luminous in consequence of the violent bombardment. the occluded air, by its impact against the oil, heats it; the oil begins to circulate, carrying some of the air along with it, until the bubbles are dispersed and the luminous points disappear. in this manner, unless large bubbles are occluded in such way that circulation is rendered impossible, a damaging break is averted, the only effect being a moderate warming up of the oil. if, instead of the liquid, a solidinsulation, no matter how thick, were used, a breaking through andinjury of the apparatus would be inevitable.

in the first place it must be observed that the bulb of the thermometer is protected so as to resist the pressure of the ocean, which varies according to depth, that of 3,000 fathoms being something like 3 tons pressure on the square inch. the new instrument is in shape like a syphon with parallel legs, all in one piece, and having a continuous communication, as shown in fig. 317. the scale of the thermometer is pivoted on a centre, and being attached in a vertical position to a simple apparatus (which will be presently described), is lowered to any depth that may be desired. in its descent the thermometer acts as an ordinary instrument, the mercury rising or falling according to the temperature of the stratum through which it passes; but so soon as the descent ceases, and a reverse motion is given to the line, so as to pull the thermometer towards the surface, the instrument turns once on its centre, first bulb uppermost, and afterwards bulb downwards. this causes the mercury, which was in the left-hand column, first to pass into the dilated syphon-bend at the top, and thence into the right-hand tube, where it remains, indicating on a graduated scale the exact temperature at the time the thermometer was turned over. the cut shows the position of the mercury _after_ the instrument has been turned on its centre. a is the bulb; b the outer coating or protecting cylinder; c is the space of rarefied air, which is reduced if the outer casing be compressed; d is a small glass plug, as in negretti and zambra’s maximum thermometer, which at the moment of turning cuts off the mercury in the tube from that in the bulb, thereby ensuring that none but the former can be transferred into the indicating column; e is an enlargement made in the bend, so as to enable the mercury to pass quickly from one tube to another in revolving; and f is the indicating tube, or thermometer proper. when the thermometer is put in motion, as soon as the tube has acquired a slightly oblique position, the mercury breaks off at the point d, runs into the curved and enlarged portion, e, and eventually falls into the tube, f, as the instrument resumes its original vertical position.

this small quarto of one hundred and eighty-four engraved pages is an example of patience in obscure letter-cutting that is more characteristic of china than of europe. the text is in german, and is fairly printed in black ink on both sides of the paper. the book is enlivened by a few illustrations which have small merit as designs. the _wonders of rome_ is an ecclesiastic’s description of the more important shrines of the holy city, with their consecrated relics. the first page of the book contains an engraving of the handkerchief of saint veronica, which, according to the legend, was placed on the face of christ to wipe away the blood that dripped from the crown of thorns, and received therefrom the impress of his features. under this design the papal arms and the triple crown, the crossed keys, and the letters s. p. q. r. the arms of the pope are those of pope sixtus iv, who occupied the papal chair from 1471 to 1484, within which period it is supposed that the book was engraved and published for german readers. the value of a self-registering instrument for solving problems, the intricacy of which is increased by the multiplicity and rapidity of the actions to be observed, cannot be better illustrated than by the success with which professor marey has thus studied some complicated actions of locomotion, as related in his extremely interesting work entitled, “la machine animale,” a translation of which has appeared in “the international series.” the action of the horse in the various paces, walking, trotting, galloping, &c., has been an endless subject of discussion, with no other data than the shoe-marks left in soft ground, and the general appearance of the animal’s movements to an observer. but m. marey—by means of elastic bags containing air, communicating the pressure through flexible tubes, so as to move little levers, which write their traces on a revolving cylinder impelled by clockwork, and carried by the rider,—has completely and finally settled all the points in dispute. it is now definitely known how the horse’s feet are placed on the ground in each of his paces, and the actual and relative time that each foot remains down. the instruments are also made to register the vertical movements of the animal, so that a complete record of its motion can be obtained. the exclusion of gaseous matter from any apparatus in which the dielectric is subjected to more or less rapidly changing electric forces is, however, not only desirable in order to avoid a possible injury of the apparatus, but also on account of economy. in a condenser, for instance, as long as only a solid or only a liquid dielectric is used, the loss is small; but if a gas under ordinary or small pressure be present the loss may be very great. whatever the nature of the force acting in the dielectric may be, it seems that in a solid or liquid the molecular displacement produced by the force is small: hence the product of force and displacement is insignificant, unless the force be very great; but in a gas the displacement, and therefore this product, is considerable; the molecules are free to move, they reach high speeds, and the energy of their impact is lost in heat or otherwise. if the gas be strongly compressed, the displacement due to the force is made smaller, and the losses are reduced.