136 
ON THE THERMOMETER. 
which are also continued beyond the boiling and freezing points ; in the latter case, after we have 
continued the divisions back 32° F. (i.e. reached + 0° F.) the degrees are numbered progressing 
backwards, as, — 1° F., — 2° F., — 3° F., &c , having the sign — (minus) prefixed; thus, — 3° F 
implies that the temperature is 3° below zero, or 35° below the freezing point of Fahrenheit’s 
thermometer. 
Unluckily, there is a complication attending the readings of the thermometer, which makes it 
difficult to compare various observations taken in different parts of the world, as several countries use 
a different scale ; the English use the scale of Fahrenheit, the French that of Celsius (centigrade), the 
the Germans that of Reaumur, and the Russians that of De Lisle. The freezing point of Fahrenheit 
is + 32, D of Reaumur 0,° of Celsius 0,° and of De Lisle + 150.° The boiling point of Fahrenheit 
is + 212,° of Reaumur + 80,° of Celsius + 100,° and of De Lisle 0°; those who wish to reduce any 
one of these scales to another, will do well to consult the article “ Thermometer,” by James Glaisher, 
Esq., in the Illustrated London Almanac for 1849. 
There are other thermometers besides those of which we have been speaking, which tell us how 
hot or how cold the ah’ has been within any given time; these are called self-registering thermometers. 
Rutherford’s construction is the best, as it is the least complicated, and consequently least likely to 
become out of working order; yet this contrivance requires two thermometers, and will even then only 
show us the extreme heat and extreme cold. The one to register the greatest degree of cold is filled 
with spirit, in which is placed a steel index, which when the spirit descends is carried with it, but 
when it again ascends is left behind. The other to register the greatest degree of heat is filled with 
mercury, which has a steel index not plunged in the liquid but placed above it; when the mercury 
rises this index is pushed before it, but when it again falls is also left behind. Thus on referring to 
the indices we at once recognise the greatest heat of the day or the greatest cold of the night. The 
first self-registering thermometer was invented by John Bernoulli of Basil; others have since been 
invented by Sixe, Rutherford, Keith, Blackadder, Traill, Lawson, &c. Of these many are now quite 
out of use, and we shall only pause to describe one which for ingenuity is not to be equalled; it was 
invented by H. Lawson, Esq., F.R.S., of Bath, and is one of a number of instruments working on the 
same machine, called the “ atmospheric recorder,” which amongst other registrations marks the tem¬ 
perature of the air every quarter of an hour, day and night. A quantity of thermometers (the tubes 
of which are bent in the middle, so that one half of each is at a right angle to the other half,) are placed 
on a balance to which is attached a long arm carrying a steel point; the balance is made to be at a 
given position at thirty-two degrees Fall., consequently a rise or fall of one degree in temperature 
upsets the balance and moves the arm either to the right or to the left, for a small quantity of mercury 
is either added in the one leg, being robbed from the other, or vice versa. A sheet of paper is made 
to move an inch an hour by clock-work across a table, over which is suspended this steel point, and a 
hammer in connection with the clock strikes the point every fifteen minutes into the paper, and com 
sequently registers the temperature fall that is requisite being to wind up the clock. 
The thermometer, an instrument so generally used and so generally useful, is but ill understood ; 
yet how all-important is this little indicator to the cultivators of stove and greenhouse plants ; it is 
true that but few plant-houses are without one or more of these instruments, on which implicit confi¬ 
dence is placed by their owners, who never doubt but that whatever temperature is indicated must 
be the true state of the atmosphere, yet unfortunately this is very seldom the case, the generality of 
thermometers are quite erroneous, and it is only by purchasing them from such opticians as Barrow, 
Watkins, Bennet, or Dollond that we may expect to arrive at the true state of the ah’. Even this is 
but one step towards obtaining a good instrument, for they must all be compared with a standard 
thermometer, which should invariably be found in the observatory of a practical meteorologist. We 
shall quote a few words on the subject from a small work recently published* :—“ To a common ob¬ 
server all may seem perfectly right; and yet when examined by placing them by the side of standards 
they are sometimes found a few or more degrees too high or too low; or what is more unfortunate, 
frequently vary in the number of degrees of error in different portions of the scale.” Instruments 
sometimes become after a time inaccurate by exposure to great variations of temperature, i. e. when 
plunged a second time into melting snow, do not point to thirty-two degrees F., but a little higher; 
careful meteorologists should therefore verify the freezing point of their thermometers every year. 
There is also a mortifying circumstance attending cheap self-registering thermometers; those for mark¬ 
ing the greatest coldness are filled with bad spirits which soon become out of order; and those for 
registering the greatest heat are even worse, for they are not unfrequently filled with oxidized mer¬ 
cury, which will soon separate in half a dozen places, and the instrument is rendered useless. 
* Prognostications of the TFeathei', by E. J. Love, Esq,, F.R,S., &c.: Longman and Co. 
