In the course of his labour, Dr. Bowditch used to say, "I never come across one of Laplace's _Thus it plainly appears_, without feeling sure that I have got hours of hard study before me to fill up the chasm, and find out and show _how_ it plainly appears." ----- Footnote 3: It is highly honourable to the sex, that the only exposition of Laplace's work that has (1848) appeared in England, is from the pen of a female--the accomplished Mary Somerville, wife of Dr. Somerville, of Chelsea Hospital. This was published under the title of the _Mechanism of the Heavens_, of which, it is observed, in the _Edinburgh Review_, "this, unquestionably, is one of the most remarkable works that female intellect ever produced in any age or country; and with respect to the present day, we hazard little in saying that Mrs. Somerville is the only individual of her sex in the world who could have written it." For this signal service to science, there was conferred upon the lady a pension of 300_l._ per annum, at the recommendation of Sir Robert Peel. ------------------ THE WORLD IN A DROP OF WATER. The microscope has shown that a drop of water though it may appear to the naked eye to be perfectly clear, is swarming with living beings. According to Ehrenberg, a cubic inch of water may contain more than 800,000 millions of these beings, estimating them only to occupy one fourth of its space; and a single drop, placed under the microscope, will be seen to hold 500 millions; an amount, perhaps, not so very far from equal to the whole number of human beings on the surface of our globe! ------------------ ORIGIN OF POST-PAID ENVELOPES. M. Piron tells us, that the idea of a Post-paid Envelope originated, early in the reign of Louis XIV., with M. de Velayer, who, in 1653, established, with royal approbation, a private penny post, placing boxes at the corners of the streets for the reception of letters, wrapped up in envelopes, which were to be bought at offices established for that purpose.